"I"nserting combos then pressing "U" will end very badly. Yeah, I know, shift-i works, but force of habit.
"I" undo bug.
Yep. Again, for now the Undo is not sophisticated enough to handle complex side-effects, such as the quest combo having their tile references shifted.
I've disabled Undo after using I; like you said, you can use Shift-I to undo insertions.