After thinking about the *very* slight warping of the wing frame I described above, I proceeded with the next step which is to glue the wing frame to the body. I decided to not to use wood glue here, but instead JB Weld because the it is critical that it hold well (the alignment tabs are IMHO way too short)... which also should un-warp the very slight warp of the wing frame.
As you can see I am holding it very rigid for a day as it dries, having checked and adjusted it so that the frame is level. Also, I put the stay tie in now (rather than after it dries) as an additional check that the wing frame is parallel and that it can slide correctly.
At this point in assembly there is no going back, or correcting anything.
I'd recommend that anybody with better experience with balsa than I consider well ahead of time how you are going to finish all these balsa parts. I had tested some Minwax on spare balsa bits and only later found warping to be a problem on the wing frame (Minwax did indeed seal the balsa, which was the point of using it). But any more warping than what I had would be the ruination of the build.
Also - after Minwax and before that warped the wing frame - I painted all the parts their recommended colors. The wings are not perfectly smooth from a lot of filing and sanding for example. This is not show quality. That will affect performance.
And I really wonder how much my finished assembly will weight against other people's.
Building was a nice exercise, and as I said above follow the instructions very carefully and read well ahead. But finishing the balsa was above my skill level... I haven't done any balsa work since my original stay in the hobby in '68-70s. I knew how to finish balsa then (and inhaled a lot of balsa dope) and do not know how now. But then you have to (re)start somewhere.
I will say again, undeniably, the design of this kit is simply brilliant! Kudos to Apogee!