1. Rainer's sort of acting as an offscreen backup to all the teams down there — she will shoot a few spells or skeletons a group's way, but since there's so much going on, she won't be able to help out enough to affect any rolls. Think of it as a plot-given distraction when considering your own plans. :)

2. That's correct! Suffice to say it will be a bit chaotic in there, but it's underground cavern that the dragon was found sleeping in! Think dark, and dank, dimly lit except for a few torches and whatever magic your team has on hand, with quite a bit of wiggle room. The entire cavern is about the size of the town square.