That should be no problem with a +18 on Hide.Since the bad guys know the PCs are inside, roll a Hide check to be able to do the extra point of sneak attack damage. The DC is 17.
Yeah, the SRD says "weapon damage rolls", so no bonuses on that account.Reminder: Don't forget about your favoured enemy bonuses for both the Rising Swords and the Loyalists. I don't think the bonuses would apply to thrown vial of acid, but I'm not sure. It says its for weapon damage.