I got round the mine problem by CLUAConsoleCreateCreaturing Yeslick and Rill and talking to them on the top level - hope I didn't miss any interesting characters or worthwhile items.
I couldn't find a CreateCreature() code for Rill, so I had to find another way.. Here's what I did (and the plus is I didn't miss any content):
1. Go to the tunnel that leads from level 1 to level 2 ans save.
2. Used Gatekeeper to alter my saved location to be on Level 2.
3. Completed the level as normal, including Yeslick (Goodbye Branwen) and Rill.
BTW, I don't know if anyone's ever found out WHY this bug happens, but if it helps, I installed FotD After I'd already started the game (I'd already done Durlag's [without going to U's Beard] in fact) and got the Assertion error, but another character I have doesn't get error (when I MoveToArea there). Among the differences between the two is that char1 has done bandit camp and char2 has not..
p.s. to get FotD on my map, I pasted the FotD worldmap into my save folder, and then did MoveToArea to every area I'd already visited.