They've been extremely tight-lipped on these details and so the real answer to your questions is that we simply don't know, and all we do have to go on are rumors. The only concrete things that they've said is that it won't be as expensive as FC housing, and we'll be able to garden.

In theory, they're designing a housing system that's meant to accommodate every single person who ever has or ever will play the game, so I'd have to imagine it'd be instanced like the rooms are in order to allow everyone to have their fair chance at owning a home. But, again, that's speculation and not anything official, which is why I share the same concerns you have. All I can say is the way they choose to handle personal housing will dictate whether or not I continue to enjoy my stay in Eorzea.