Don't know about 1st question, but on 2nd and 3rd: the balance with bots is a whole different thing than the balance between human players, so that's why there are differences between sp and mp maps. Also, bots can't walk/run/jump through various paths where humans can, that's another reason for the differences. Considering the AT guns: not sure, but bots might sit endlessly in position 1 not being able to fire a shot ever.