Short question: When I have a stack on an underground entrance and stacks next to it...and another stack comes from the other side: do only these two stacks fight or is the one entering the cave entrance fighting against all stack that are around the target hex?
Not so bad. That is hardly repeatable in real game. This behaviour is tricky and very unstable. You will see that if you try to repeat. In real game in 19 out of 20 attempts you will just lose the drone or both, not even worth to try imho.
For me it was just academic interest, what will be the weakest possible army to take ruins.