There's an elevator hidden behind a false wall in the Zhent hideout that takes you down to the underdark stash. It's a much better place to fight the Minotaurs from plus you can see how their patrol makes more sense as guards from there.
As for why the Zhents are there, aren't they the major suppliers of surface slaves and goods to the underdark now that the trade with Thay has collapsed? (because would you buy food from zombie farmers, it's a bit icky, and probably contaigous)
Like I said, that one's a bit more believable, but the problem I have with that is that the game makes a point to make it seem like the only way in or out of that area is either via the lake or through the temple. Even if that location has been kept secret, how are all of these other people getting in there? How did the gnomes get down there? How did the dwarf merchant and the Hobgoblin and the mind flayer?
