If a portal between material and ethereal planes is created and then destroyed, and then created again in the exact same place in the material plane, would the new portal link to the same location in the ethereal plane as the first did? 

If a new portal to the ethereal plane is opened in the material plane 20ft away from the first portal, would the portal in the ethereal plane appear 20 feet away from the first portal in the ethereal plane also?

Finally, whatever the answer to the above, is it true for all links between all planes, or is it specific to the plane?

All three of those quesrooms have the same answer:

Not necessarily.

To expand on what Sposta said.

There's not necessarily hard-and-fast rules for how portals connect. If you look at the gate spell, you can see the description of how you place a portal and where it might link to. Basically, if you are creating a replacement, and want it to link to the exact same spot, you can probably pull that off, but you could also choose to have it connect to an entirely different location on the target plane, assuming the god who lives there is OK with it.

Thanks. That's what I expected, but I was confused by this sentence on the Transitive Planes . To me that makes it sound like there's some fixed mapping between one plane and another.

Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

Based on that it sounds like the Shores of the Ethereal plane, if not the entire plane, are a special case. 

I would rule that under most circumstances yes, you can go back and forth to the ethereal plane and if you start on the same location in the material plane, you get to the same location on the ethereal plane.

Otherwise you run into problems with the spell blink and ghosts.

Quote from CrowXeon >>

Thanks. That's what I expected, but I was confused by this sentence on the Transitive Planes . To me that makes it sound like there's some fixed mapping between one plane and another.

Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

 

Based on that it sounds like the Shores of the Ethereal plane, if not the entire plane, are a special case. 

 

 I see what you are saying, but just because there is a corresponding point doesn't mean the portal needs to open at that point. It can, but it can just as easily be somewhere else.