1. Regions with difficult terrain should not be allowed to hold more than 6 (combat) units per side. Right now, the game correctly gives an error message when you try to move a stack larger than 6 units into a difficult region. However, it still lets you move several stacks that are smaller than 7 units into the region, and even lets you combine them. So you can easily move an unlimited number of small stacks into difficult regions. For the attack, however, you're really limited to 6 units, as you can only attack with a single stack. I don't think that the work-around (move multiple small stacks instead of one big stack to circumvent the "6 units" limit) is intended? I think it's a bug.
Also, stacks can retreat into difficult regions where there is already another stack, potentially increasing the number of units in the region over 6.