I'll probably give things another try after I get some rest.
I tried to come up with some build mode stuff that might at least make it easier for sims to move around provided that there path is blocked when using stairs or platforms.
Things that have helped:
Stairs: If you can remove the rooms directly above the stairs, you should have a house that your sims can get around in again.
If you use fences to wall off the ledges at the top of the stairs: Replace the fences with half walls and make sure the space isn't closed off by fences where the top of the stairs meet the upper floor.
Stairs Outside: The stairs can still have a roof above them provided there is an entire floor between the top of the stairs and the roof. If you can put the stairs outside, that'll work even better.
Patios are your friend: You can use patio decks as a connector between a room and stairs outside a room. This appears to work.
Platforms: As long as the platforms are half the height of the room walls, they appear usable.
Using platforms as a substitute for stairs might only work outside a room.
Spend less time in Build Mode: I'm going to test it later, but I think I encounter the crashes more often as I spend more time in build mode. The sound almost always cuts out while in build mode and did not return after it cut out. Crashes occurred shortly after a loss of sound.
This part may be difficult to do if stairs that used to be usable need to be edited. I'll be experimenting with this next.
Reinstall the game: This is a huge pain, but I think it helped significantly. When I reinstalled the game, I was able to do things in the game a little easier. I'm going to reinstall the game again while I take a nap or watch Netflix. I attempted the reinstall when the pack was released after preordering. I'm not sure if that affected the game. To remove the variable, I'll reinstall the game knowing that the pack has already been released.
