You could perhaps solve some of the pathfinding issue by setting the lower volcano to erupt at a later stage - that way there is less of a choke point for the enemy AI.
Yeah, I tried that, which reduced the probability of its units getting stuck, but not by much. I'd have to do the dirty work of cleaning up the map in the editor to make a lot of the chokepoints less complex. Alternatively, replace OP2's pathfinder with a better one based on a graph search like D* Lite, or some other A* variant, which I actually was looking into at some point; but, the pathfinder code disassembly isn't well-documented, and I don't know which function exactly is OP2's graph search algorithm, nor which one generates the graph to begin with (I'm assuming it's not using the raw map data and uses a quadtree or similar). Which would be cool, and way more work than just trying to clean up the map.
Compared to StarCraft 1's pathfinder (which came out 6 months after OP2), we should be glad with what we have, funnily enough.