As it turns out, it was a good thing I didn’t put any real effort into this, as my players opted *not* to interfere with the Zhents and just left town the next day.
As agents of the Lords’ Alliance, they filed a report on the situation in Mornbryn’s Shield when they got back to Yartar and then moved on to the next quest: heading into the High Forest in search of Shadowtop Cathedral. (Although they got sidetracked in Calling Horns and are now heading into the Evermoors to hunt trolls.)
That said, I do appreciate all the help everyone gave me in this thread, and I will keep the suggestions in my back pocket in case they prove useful down the track.