Remove the use of grenade until you can make that happen then. Cause now you introduce something which doesn't supposed to work like it does now.
If you use these spells in a 1on1 battle they might look and feel fine. But if you use them in a 20on20 battle you just can't control anything anymore. It's just a splash fest of damage and minions flying around which you can't micro anymore.
You say things are bugs, but what if Bullfrog did it on purpose cause they also concluded it doesn't play great if everything splashed around?
The grenade is a great spell for 1st person mode, but an army auto spamming them is just not going to work even after changing the whole thing. (which is something A custom mapmaker can do, but you shouldn't do to the original game)
Fixing the path finding should be one of the higher priorities indeed. Next would be the rooms spamming (workshops, barracks mostly) of the AI. Which can be (temporarily) fixed by limiting the amount of tiles they can use.
On a different note, what is the AI looking for when it digs around rocks? Sometimes they mess up their whole dungeon with digging around the rocks. They know the map layout so "looking" for things in that way doesn't really do anything for them.