The fire idea has merit (wasn't that strategem used with some success in the seige Dave previously alluded to in Orbaal?), but I wonder if one of the outbuildings would be a better target. More likely to be wooden, for one thing, and more or less likely to achieve the intended effect. And perhaps Bevan, as a Laranian herself (albeit a tepid one), would be more at ease in not compounding the major sins of killing high priests with the minor sin of firing the place of worship
As for a disguise, if the visit to the service is kept as an option, perhaps hair can be dealt with via whatever headgear happens to be in fashion among the female commoners. If the plan is to dress down, as it were, which I think is the sensible way to go, then the woolen attire may be supplemented by some form of napkin upon the head
BTW, if we take this route, it would seem sensible for Bevan to attend the service, as she will know when to kneel. Ewen and Imarë are likely to give the game away by looking clueless, unless attending with Bevan I suspect.
The notion of snatching some poor defenseless person for the sake of obtaining info about the details of the abbey should certainly be on the table. We will need to act fairly fast on the heels of taking that move, of course, but then speed seems to be the agreed-upon method anyway.
And I have, at this point, taken counsel from my peers and have abandoned my concerns about meeting Palgren in Avertu. 'Nuff said.