This build has progressed faster than I expected. A few minutes here and a few minutes there and I've made it pretty far.
I will still defer many details of the build/prime/paint sequence, but I did (after going back and forth a few times) decide to mount the rear fins before priming. On my first few BAR builds I primed all parts before any assembly, but I'm modifying that and now leaning towards assembling anything I can before priming. Basically, anything that I believe I'll be able to sand effectively after assembly gets assembled first. The rear fins, minus the tubes, being TTW mounted and all, should be easy to sand in place.
For each fin, I first checked tab height. One of the three needed to be knocked down just a bit, and then it was snug; the others were good as-is. Then I glued each fin against the, uh, "left" edge of the slot, which was the side that I originally marked with the fin-marking guide, and applied some Titebond II to that joint as well. Here's one in the comfortable embrace of my 3D-printed alignment jig:
I then had a look at the gap on the other side. Here's the biggest one:
I stared at it for a while, though of ways to fill it, and then just said "fillet". I applied my usual fillets with Titebond NRND and just let it cover over the gaps. The end result feels plenty strong, don't really foresee any problems. In hindsight, I probably could have done a better job trying to force some glue *into* the gap but I don't think it will matter. The rear fins are attached.
I haven't yet succeeded at getting a good fillet picture, if I do get one I'll add it later, but hey, they're just fillets. [edit: no fillet pictures forthcoming. However, they look good and the fins seem really strong]
And now, somewhat amazingly, I am ready to prime. Which means a nice, painful break in the action while I wait for an opportunity to go outside and do some spraying. In the interim I'll write up the sequencing.