Well, I finally managed to get something done – and that something turned out to be the next story in Tremors: The Subtext, Showdown, which I’d been poking at for a while and couldn’t finish. 
Turned out what it needed was more violence, some call-backs to previous stories in the series, and a bit of extra schmoop to wrap up the ending a little more nicely. Apologies for it not being a Christmas story, but in spite of all the violence and (cyobactyl) killing it still has a very sweet happy ending, so there is that.