So first of all, the fact that you only spend a spellslot when preparing the summon and not for the summoning itself makes this SO flexible (if you're rich).

Secondly, it says that it's available specifically for Eldritch Knight and Arcane Trickster but it's a 6th level spell and neither of them get any slots higher than 4th level. Is this a bug? Or is there a legitimate way for these subclasses to cast this?

Multiclassing can give you higher level spell slots. Normally you can't gain access to higher level spells this way, but instead you can just cast lower level spells at a higher level. However, the rules allowing certain classes to swap out spells instead of learning new ones may overrule this.

If your character has 6000+ gold to spare (by 11th level it's feasible, depending on the DM and your spending habits), this spell can work wonderfully when combined with Leomund's Secret Chest. My Warlock character has once surprised the party with this combination when we were imprisoned, with a 10-step plan to happiness:

1. (Optional) Purchase scrolls of Leomund's Secret Chest and Drawmij's Instant Summons, if your class can't cast them inherently. Or ask a party member who can.
2. Dish out 5050 gold to cast Leomund's Secret Chest.
3. Stuff it with anything your party might need if their primary gear gets taken away (component pouches, weaponry, armors, holy symbols etc.).
4. (Optional) Equipping the chest with highest quality lock, an Arcane Lock spell, and a Glyph/Symbol spell for a good measure can't hurt if you want to feel safe.
5. Dish out 1000 gold on a sapphire to cast Drawmij's Instant Summons on the tiny chest component for Leomund's Secret Chest. Leave the tiny chest somewhere VERY safe.
6. Embed the sapphire in your mouth, as a part or a replacement for a tooth.
7. When imprisoned, tell your barbarian friend to deck you in the face (to get the sapphire out).
8. Crush the sapphire to summon the component for Leomund's Secret Chest.
9. Use the component to summon the actual chest.
10. Rub your hurting face while being praised by the party for your thoughtfulness.

Once again, throwing money at your problems makes them go away!

It specifically states you must use a different sapphire each cast.

The spell is prohibitively expensive, it would be manageable if the sapphire was not crushed in the process. 

I have found only one good thing about this spell, and that thing is that it's a touch spell. Now many would argue this is a problem but that's what makes it devilishly fun. If their are incredibly expensive items in a store far over 1000 Gp, then don't fret because you can do something absolutely insane. Have your familiar (preferably with invisibility cast on it, your a wizard so you SHOULD have it) touch the super expensive magic item and cast the spell. Have your familiar then stay in the magic shop and watch the item so you can make sure it isn't moved or being held by the shopkeeper. Before you crush the sapphire and take the ridiculously expensive item see through your familiars eyes and check on the item, if the item is still there, crush the sapphire and claim your item. (The only practical applications for this spell is if your spellbook gets destroyed and your copy is miles away so you can't reach it in a timely fashion. The other application as seen here is using your familiars ability to utilize touch spells and using said affinity to steal crap)

Aside from stealing or scamming shenanigans, the only practical use I see for this spell is insurance. You can link your most valuable magic items to sapphires in case they are lost or stolen. A pretty niche and expensive spell all in all. That said, a sapphire with this spell cast on it could be a cool thing for a DM to sneak into a party's inventory... though with a high chance of them missing it entirely.

I figured it out. This spell is for Necromancers/Liches that have a Clone spell or a Phylicteriy prepared in a safe place.

by gum, you  don't want to be killed and come back with your most powerful items in the hands of the party!

Stop giving the spells different names, or make it possible to turn off. Please. I don’t want this nonexistent spell to keep popping up whenever I try to look at a list of spells.

Quote from Platyposeidon_Z >>

Stop giving the spells different names, or make it possible to turn off. Please. I don’t want this nonexistent spell to keep popping up whenever I try to look at a list of spells.

 

 D&D Beyond isn't "giving the spells different names." Wizards is. This is the Basic Rules version. The named version is from the PHB.

The spell does NOT say that the material component is consumed, so presumably you could cast Mending on it to get your gem back to form after crushing it. 

The spell DOES say each time you cast the spell you have to use a different sapphire. What that means is up to interpretation. It could mean:

A. Each item needs its own gem, so you can’t have a bunch of things linked to one gem. 
B. Each gem can be used only once, so after summoning something and then restoring the gem with Mending you have to go back to town and trade your 1k gp gem a different 1k gp gem. 

The answer is up to your GM, but I lean towards interpretation A. The problem with B is that the spell isn’t very powerful and you run into the chance that 1k gp sapphires randomly will not work for this spell (because someone used it already at some point in history). There really is no need to limit it further. 

How does this spell work in conjunction with wish? does a sapphire appear as a side effect so that we can crush it? maybe all you have to do is call its name?

The main use for this I've found is for necromancers with a clone spell set up.

Cast it on your magic items and bag of holding, leave the sapphires next to your inert clone, and as soon as you get killed, summon all of your items back to you. While it might give away you're still alive, most adventurers will just assume that the items died with you or something as long as you're quick about it

So, is this based on Drawmij's instant summoning's from the players handbook?

So....

What happens if a genielock casts this spell onto their vessel.

And then summons their vessel while inside their vessel?

Bag of holding is 15 lbs. The weight limit for this spell is 10 lbs.

Bag of Holding may weigh 15 pounds, but Handy Haversack weighs 5 pounds, and Portable Hole has no weight set for it.

Reading this after coming up with a cool concept only to realize a pike is probably more than 6 feet long (Mithril, so less then 10 lbs.) and won't work with this hurts. But **** it I'm homebrewing the crap out of my campaign from the ground up.

For what it is this isn't a great spell IMO; fine for a Wizard to write into their spellbook for use as a ritual, but I wouldn't want to cast it using a spell slot.

While by the time you have 6th-level spell slots 1,000 gp may not be all that costly it's just hard to imagine many situations where this spell is more useful than leomund's secret chest or dimension door (to simply bamf out of your prison cell and into/near to the confiscated equipment room).

You can kind of engineer scenarios to make it more useful, for example giving a "gift" to a creature you want to be able to track, so that when you crush the sapphire you can find out where they are, but it'd have to be something you are certain they'd never remove, because otherwise you'll just summon it if they took it off/put it down. The only other idea I've had is if you can spend the time with an item you want to steal from a secure location, e.g- by social stealthing your way in to see it, then you can simply summon it out later, but the fact you need to touch it for 1-11 minutes makes this a super suspicious thing to do, and a Wizard has so many other, better options (just grab the item and dimension door somewhere secure where you can escape or disguise yourself).

I dunno, it 's a very strange spell, and a lot weaker than similar lower level features such as a Warlock's ability to summon a pact weapon.