its very simple.
1. as soon as your interior starts to CTD on exit, move your load door (the door only, not the teleport marker) outside of your walled area. put it out somewhere into the void so that its far enough away that when the door opens you dont hear it opening.
2. create a "dummy door" in the place your original door was. I personally prefer to use an activator object that uses the load door's nif, and set the Activate Override to say Open, or Travel TO or whatever you want.
3. attach the script i posted to the activator dummy door
4. create the purgatory cell - go to your world tab at the top of the CK, and select Cells. right click in the cell window (left hand column) and create new. name this something specific MyModPurgatoryCell etc
5. Drop an xmarker header into it and name the reference PurgatoryMarker
6. go back to your dummy door and double click it to edit the reference window
7. go to the scripts tab and point your PurgatoryMarker property to your xmarkerheader (or just hit autofill if you named it the same)
8. point the DoorRef property to the real load door that you pushed off to the void space.
the advantage to this solution is so that NPCs can find their way to and from your cell on their own.
for example if you release one of your followers inside your interior, they cant find their way out unless a navmesh has a teleport marker that eventually links to tamriel
also if you have NPC's that use packages that travel to and from your interior, they wont be able to run their packages unless you have a properly linked navmesh (which is why it is important to leave your door teleporter on the oringial spot on the navmesh)
After some testing I don't believe it's possible to make NPCs use my system, it works for the player but it doesn't work for NPC's because my code lacks the part where the player activates the teleport (I think this is the problem).
Gonna put your solution in the wiki, if you have a problem with that just delete it all, thanks.
With my testings I've found something funny, the npc while following us doesn't need a navmesh at all, funny.