Wall of Force simply says it blocks physical things. Forcecage, on the other hand, blocks spells when created as a box (rather than a cage). "A prison in the shape of a box can be up to 10 feet on a side, creating a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area." That suggests that Wall of Force doesn't block spells, because Forcecage says it does, and Wall of Force does not say it does. If the Clear Path rules were sufficient to block spells from being cast through a Wall of Force, Forcecage wouldn't need that text.
This text is clarifying the differences between the cage-shaped Forcecage and the box-shaped Forcecage. The box-shaped Forcecage creates Total Cover (hence, spells cannot be cast into or out of it, as mentioned in the description). The cage-shaped Forcecage doesn't do this because there are spaces between the bars -- this shape would only create Half Cover and spellcasting is always able to bypass Half Cover.
(What's the point in differentiating between 'a space you can see' and 'a space within range' (without requiring you can see it). Spaces are generally not invisible.)
In general, these spells are capable of being cast at a creature that you can only hear (but cannot see) such as a creature that is lurking within an area of Darkness or a creature that is located within a Fog Cloud and so on.
All your examples are physical things being redirected by other physical things. Yes, a physical thing blocks a physical thing.
Show me an example of a non-physical thing treating a physical thing as a obstacle.
There is no need to come up with such an example since that's not how it works. A physical obstacle is an obstacle by its very nature. It exists regardless of what is interacting with it. Some phenomena might be capable of ignoring or bypassing obstacles, but those obstacles still exist. If a ghost-like creature walks through a wall it doesn't mean that the wall is not an obstacle -- it is. Physical walls are obstacles:
Walls, trees, creatures, and other obstacles . . .
Once it is established that such an obstacle exists and therefore provides Total Cover, the next question is -- does the phenomenon care if there is Total Cover or not. Often it does matter. Sometimes it does not matter. In the case of the spellcasting procedure between the spellcaster casting the spell and the point of origination of the spell effect, Total Cover always matters, by rule:
Range
A spell’s range indicates how far from the spellcaster the spell’s effect can originate, and the spell’s description specifies which part of the effect is limited by the range.
A range usually takes one of the following forms:
Distance. The range is expressed in feet.
. . .
Targets
A typical spell requires the caster to pick one or more targets to be affected by the spell’s magic. A spell’s description says whether the spell targets creatures, objects, or something else.
A Clear Path to the Target. To target something with a spell, a caster must have a clear path to it, so it can’t be behind Total Cover.
Also, if you're right, Sending doesn't work as described. It never says it ignores intervening obstacles, but if it doesn't, it can't actually deliver a message anywhere on the plane. (If you're on an Earth-sized globe, for example, the planet itself becomes an "obstacle" after ~5km). Clearly sending ignores 'obstacles' to work as described. And why might it do that? Oh yeah, because its not physical and so those things aren't obstacles. (And you almost never have a clear path to other planes). I'm sure that's not the only blindingly obvious example.
Edit:
Spells that definitely don't work as described if you need a clear path and physical things count as obstacles: Sending, Clairvoyance, Telepathy, Dimension Door
Spells that probably don't work properly if you need a clear path and physical things count as obstacles: Tsunami, Storm of Vengeance, Hallucinatory Terrain, Mirage Arcana, Control Water. Many of these have long ranges (1 mile), and affect large areas. If you have to have no obstructions to any part of the area, they're going to be unusable. but even requiring no obstructions to one part of the area is going to be hard for spells with a range of 1 mile. (Control Water is an interesting one, because a 100' cube of water, even if you include the visible surface, is going to itself be an obstacle to much of the water affected. There's no line of effect to most of the target).
I agree that a few of these spells are problematic to the point where they might actually need errata in order to make them so that they actually function correctly (as intended) within the framework of the rules as written.
But there are a great many spells in the game which somehow interact with things that are very far away, but the author correctly gets around the Clear Path rule by designing the spell such that it has a Range of Self. Lots of Divination spells work like that, such as: Augury, Commune, Contact Other Plane, Divination, Find the Path, Scrying and so on.
Let's quickly look at the spells you've mentioned:
Sending is an example of a spell that is probably intending to create a Specific Vs General Exception within its own spell description, but it falls short of doing so. Or, at least, it relies on the reader making a very generous interpretation that the text creates such an exception when it really doesn't. The text in question is: "You can send the message across any distance and even to other planes of existence". Does this create an explicit Exception to the Clear Path rule? I'm not so sure, but it's open to interpretation. It's unclear why the author didn't just make this a spell with a Range of Self like so many other similar Divination spells already have. The text already describes the target creature such that it does not have to be "within range", so this would be an easy fix. In my opinion, the spell should be tweaked with errata to either have a Range of Self or to make it more explicitly clear that the Clear Path rule is bypassed by the spell description. Whether or not that's necessary or if the current wording is enough to function properly is open to interpretation. Obviously, in actual play we all know what is intended in this case, so we would play it that way.
Telepathy is very similar to Sending. It would benefit from similar errata. It doesn't do a good enough job of making an explicit Exception to the Clear Path rule, and it could easily be fixed by turning the spell into a Range of Self spell since the target creature mentioned in the description does not have to be "within range".
Mirage Arcane is an interesting one since it has a range of "sight" which isn't well defined, but we can pretty easily guess at the intent. I take this to mean that if you can see it then it is within range. Tactically, this spell would benefit by the spellcaster getting to some sort of high ground so that he can see much farther into the surrounding area. Many other spells such as the ones with a range of "1 mile" would benefit in this same way. These spells generally work as intended. In this case (Range of "sight"), if there is an invisible barrier, such as a Wall of Force, in the way of the target then the Clear Path would be interrupted even though you can see the target beyond. Just because something is within range doesn't mean that there's a Clear Path. Both must be true at the same time.
Skywrite works similarly to Mirage Arcane.
Project Image, as discussed before, creates the explicit Exception.
Of the "1 mile" Range spells, I would say that Meteor Swarm, Mighty Fortress, Storm of Vengeance and Tsunami all function as intended.
Clairvoyance, on the other hand, doesn't appear to work as intended. This spell needs errata. There is not enough in the spell description to justify an explicit Specific Vs General Exception to the Clear Path rule, and the spell is clearly intended to target a location "within range" of 1 mile, potentially behind obstacles ("behind a door, around a corner, or in a grove of trees") so turning this spell into a Range of Self spell wouldn't make as much sense, even though it's a Divination spell. On the other hand, the spell does create a "sensor" that is very similar to the one that is created by the Scrying spell -- in both cases the sensor is created at a distant location, but the Scrying spell has a Range of Self. So, perhaps that is the proper errata for the Clairvoyance spell after all.
Dimension Door suffers from a similar issue as Sending and Telepathy where it seems like the spell description is attempting to state a Specific Exception for the Clear Path rule, but the wording falls short. Once again, the simplest erratum for Dimension Door is to simply change it so that it has a Range of Self. For comparison, the Misty Step spell and the Teleport spell both have a Range of Self.
Hallucinatory Terrain works as intended.
Control Water also works as intended. As an aside, that spell should include the phrase "within range" in the first sentence, but that's a separate issue.
First of all, a body of water is generally not considered to be an obstacle. It's not an object and it doesn't provide Total Cover (for example, there are Underwater Combat rules whereby it is possible to make non-magical ranged (missile) attacks against a target creature).
The other thing about Control Water is that it creates a Cube AoE that controls water within it. A Cube AoE has its point of origin on one of its faces:
A Cube is an area of effect that extends in straight lines from a point of origin located anywhere on a face of the Cube.
So, even if the water did create an obstacle, you would only need a Clear Path to the edge of your prospective Cube that you intend to create. Once the spell effect originates, the spell effect expands to fill the Cube. You don't have to have a Clear Path to the entire cube, only to its point of origin.
Anyway, the main takeaway from all of this is that just because a few spells exist in the game that are poorly written and require errata doesn't mean that the general Clear Path rule doesn't exist and/or that it doesn't say what it says and/or that it doesn't work how it works. The Clear Path rule applies to all spells in the game. It is a general rule for spellcasting.
This text is clarifying the differences between the cage-shaped Forcecage and the box-shaped Forcecage. The box-shaped Forcecage creates Total Cover (hence, spells cannot be cast into or out of it, as mentioned in the description). The cage-shaped Forcecage doesn't do this because there are spaces between the bars -- this shape would only create Half Cover and spellcasting is always able to bypass Half Cover.
In general, these spells are capable of being cast at a creature that you can only hear (but cannot see) such as a creature that is lurking within an area of Darkness or a creature that is located within a Fog Cloud and so on.
There is no need to come up with such an example since that's not how it works. A physical obstacle is an obstacle by its very nature. It exists regardless of what is interacting with it. Some phenomena might be capable of ignoring or bypassing obstacles, but those obstacles still exist. If a ghost-like creature walks through a wall it doesn't mean that the wall is not an obstacle -- it is. Physical walls are obstacles:
Once it is established that such an obstacle exists and therefore provides Total Cover, the next question is -- does the phenomenon care if there is Total Cover or not. Often it does matter. Sometimes it does not matter. In the case of the spellcasting procedure between the spellcaster casting the spell and the point of origination of the spell effect, Total Cover always matters, by rule:
I agree that a few of these spells are problematic to the point where they might actually need errata in order to make them so that they actually function correctly (as intended) within the framework of the rules as written.
But there are a great many spells in the game which somehow interact with things that are very far away, but the author correctly gets around the Clear Path rule by designing the spell such that it has a Range of Self. Lots of Divination spells work like that, such as: Augury, Commune, Contact Other Plane, Divination, Find the Path, Scrying and so on.
Let's quickly look at the spells you've mentioned:
Sending is an example of a spell that is probably intending to create a Specific Vs General Exception within its own spell description, but it falls short of doing so. Or, at least, it relies on the reader making a very generous interpretation that the text creates such an exception when it really doesn't. The text in question is: "You can send the message across any distance and even to other planes of existence". Does this create an explicit Exception to the Clear Path rule? I'm not so sure, but it's open to interpretation. It's unclear why the author didn't just make this a spell with a Range of Self like so many other similar Divination spells already have. The text already describes the target creature such that it does not have to be "within range", so this would be an easy fix. In my opinion, the spell should be tweaked with errata to either have a Range of Self or to make it more explicitly clear that the Clear Path rule is bypassed by the spell description. Whether or not that's necessary or if the current wording is enough to function properly is open to interpretation. Obviously, in actual play we all know what is intended in this case, so we would play it that way.
Telepathy is very similar to Sending. It would benefit from similar errata. It doesn't do a good enough job of making an explicit Exception to the Clear Path rule, and it could easily be fixed by turning the spell into a Range of Self spell since the target creature mentioned in the description does not have to be "within range".
Mirage Arcane is an interesting one since it has a range of "sight" which isn't well defined, but we can pretty easily guess at the intent. I take this to mean that if you can see it then it is within range. Tactically, this spell would benefit by the spellcaster getting to some sort of high ground so that he can see much farther into the surrounding area. Many other spells such as the ones with a range of "1 mile" would benefit in this same way. These spells generally work as intended. In this case (Range of "sight"), if there is an invisible barrier, such as a Wall of Force, in the way of the target then the Clear Path would be interrupted even though you can see the target beyond. Just because something is within range doesn't mean that there's a Clear Path. Both must be true at the same time.
Skywrite works similarly to Mirage Arcane.
Project Image, as discussed before, creates the explicit Exception.
Of the "1 mile" Range spells, I would say that Meteor Swarm, Mighty Fortress, Storm of Vengeance and Tsunami all function as intended.
Clairvoyance, on the other hand, doesn't appear to work as intended. This spell needs errata. There is not enough in the spell description to justify an explicit Specific Vs General Exception to the Clear Path rule, and the spell is clearly intended to target a location "within range" of 1 mile, potentially behind obstacles ("behind a door, around a corner, or in a grove of trees") so turning this spell into a Range of Self spell wouldn't make as much sense, even though it's a Divination spell. On the other hand, the spell does create a "sensor" that is very similar to the one that is created by the Scrying spell -- in both cases the sensor is created at a distant location, but the Scrying spell has a Range of Self. So, perhaps that is the proper errata for the Clairvoyance spell after all.
Dimension Door suffers from a similar issue as Sending and Telepathy where it seems like the spell description is attempting to state a Specific Exception for the Clear Path rule, but the wording falls short. Once again, the simplest erratum for Dimension Door is to simply change it so that it has a Range of Self. For comparison, the Misty Step spell and the Teleport spell both have a Range of Self.
Hallucinatory Terrain works as intended.
Control Water also works as intended. As an aside, that spell should include the phrase "within range" in the first sentence, but that's a separate issue.
First of all, a body of water is generally not considered to be an obstacle. It's not an object and it doesn't provide Total Cover (for example, there are Underwater Combat rules whereby it is possible to make non-magical ranged (missile) attacks against a target creature).
The other thing about Control Water is that it creates a Cube AoE that controls water within it. A Cube AoE has its point of origin on one of its faces:
So, even if the water did create an obstacle, you would only need a Clear Path to the edge of your prospective Cube that you intend to create. Once the spell effect originates, the spell effect expands to fill the Cube. You don't have to have a Clear Path to the entire cube, only to its point of origin.
Anyway, the main takeaway from all of this is that just because a few spells exist in the game that are poorly written and require errata doesn't mean that the general Clear Path rule doesn't exist and/or that it doesn't say what it says and/or that it doesn't work how it works. The Clear Path rule applies to all spells in the game. It is a general rule for spellcasting.