Authorities in the town itself are obviously compromised, so you'd have to go further afield and either appeal to the leaders (mayor, sheriff, council, etc.) of a larger town nearby or to a local lord and see if they can be motivated to get involved. The Duke of Ghislain would be the top of the noble hierarchy here, but there are probably a few in the lower tiers who have some level of authority/influence in the region, though many are not currently in residence. If that's the route you want to go we can make up more specifics, or if you'd rather bypass them they can just be in Val Royeaux or in the army or something and not available.

The Inquisition and the Chantry, like you mentioned, have a presence in the region that definitely comes with some authority, especially right now. They'll have a hierarchy of people overseeing operations in the region and in sub-areas, so they could take this to the local captain or sister, or up the chain past them to the people overseeing the whole Fields operation. (Doing this could look like conceding that Riftwatch answers to them and that they have the right to decide things that Riftwatch doesn't, so that's something to bear in mind, but on the other hand who among the DHs would want to touch this business with a ten foot pole.)

Obviously this is all a bit vague because we don't have NPCs ready to go here, but if one of these options interests you and you think your characters would reasonably know some facts already about the people they'd be applying to, I can make some up.