1) I think this depends on the AMPC. Most monsters probably fly under the radar because their presence doesn't impact the Dark Lord. Strahd doesn't particularly care about some wererats or werewolves hiding in sewers or caves. He has minions to deal with that (which incidentally includes the Vallaki Garda and, by extension, everyone who helps the Garda). The idea isn't to give vast tracts of real estate over to some new constant threat, but to give AMPCs a little bit of flexibility in setting up a secret lair.
You could have a few set piece "rentals" which AMPCs, e.g. Isolated Cave, Abandoned Manor, Blasted Windmill... you get the idea. Every monster wants a big climactic battle in their lair and this is a way to see that done in a way that potentially doesn't require DM involvement (more on that in point 3).
2) Could it be abused? Possibly. We're only speaking in hypotheticals here, so the idea of having the entrance to the lair as a transition portal placeable item is still just an idea on a drawing board. So as an idea on a drawing board, here are some thoughts on how it could be implemented fairly:
- The idea behind making the lair's entrance location entrance dynamic is to remove predictability and prevent server knowledge from being a big asset in knowing where to find secret lairs. Every player who's done a lot of exploring on the servers probably has a list in their head of cool places to have a lair, and that's naturally the first places that many veteran players will look.
- Throw in the fact that any evil lair needs carpentry placeables, right now as is, evil lairs are pretty easy to find. Much easier if you could just find a small quiet area of the map to drop a transition portal placeable (like the ones that DMs set for events) that takes you to the Abandoned Cave or whatever.
- These lair transition portals should most certainly NOT be plot-lockable: they exist to be found by other PCs, they're just levelling the Hide & Seek game a bit.
- These lair transition portals should be fixed once an AMPC places them, and require a DM to un-fix them: after all, the idea here is not to give AMPCs a magic teleporting sanctum sanctorum, but a stage for a final showdown.
3) Right now, it's possible for DMs to create a transition portal to an un-used "backstage" map to set it up as a lair, but DM time is extremely limited and most of these things may not last a reset. The idea would be to automate and standardise this possibility into Lair Rentals.
Of course this is all pie-in-the-sky thinking until and unless someone wants to code this.