EDIT:
It works!
Problem 1:
I finally got to logging layers (+DS list with tilemaps stored with room_pack_store_tilemaps) -
This cleared up some confusion: Tilemaps don't have to be assigned to layers.
So I figured out how to get tilemap names and came up with this code:
It runs whenever room is loaded and stores correct tilemap positions in tilemap_list.
Now I need to create two tilemap lists, because I need two rooms loaded, and whenever room is loaded, it creates new tilemaps - currently I store just the last loaded room's tilemaps - the other room's collisions stop working.
If two tilemap lists will work, it'll be done. I hope I'm not doing something horribly wrong.
This is the code I use to log layers, just in case:
Problem 2:
I managed to fix it - the problem was that loaded room object create events fails to set layers, probably because of how room loading works.