I had this problem and finally found a way to get "unstuck"... The tomb is poorly designed and, unfortunately, if you don't have Escape Dust (or whatever it's called lol), you will probably stay stuck.
Good news is there is a cheat that you can use to get unstuck (I know it sucks to use a cheat in a tomb, but I haven't found any other way).
If you find yourself stuck behind the stairs after you activate the switch on the wall follow these steps:
-Bring up the cheat box (Ctrl+Shift+C) and type in testingCheatsEnabled true
-Hold down shift and click on a random tile, then hit the option "Build on this Lot" (for some the option "Teleport" will come up, but that was not the case for me. If "Teleport" comes up then use this instead.).
- It should let you go into buy mode in the tomb. Click on the Dart Trap to the left of the stuck sim. Delete the Dart Trap
I hope this helps for those who are trying to find out how to get past this frustrating problem. I hate to delete a trap in a tomb, but it was either that or go back home when I have 5 days left on my trip.