Cass might hang back out of sight as much as possible on the off chance their scheme doesn't work.  (That and I'll be gone Sunday so it's just as well honestly.)  But my idea is more tactical in that if our two 'Lathandar priests' aren't able to talk their way into taking over, then we can hit them in waves, with my sorceress acting as ranged artillery of sorts.  I have the Shield spell handy for that reactionary bonus defense, so that SHOULD help somewhat if things go poorly.  It's very unlikely that they'll let you take over though.  At the very least you can get close to the cart and set them up for ambush later.