Well I think that explains the spin: the engine pointing too far to the left was offset by aileron trim. And the excess decollage/nose down attitude of the motor offset by a lot of down elevator trim. When the engine quit, the plane was left with ailerons offset to turn right and elevator down = spiral nose dive.

I'll try it again with the motor pointing 2 deg right and 2 deg down, and see how that goes. Then if I still need a lot of down trim on the elevator, I will adjust the angle of incidence of the elevators by taking a narrow wedge out the the tail section of the fuse.

Worst that can happen is that I retrieve my hardware from a pile of matchsticks and move on. Anyone suggest a good sport bellylander (our hay field is too bumpy for wheels) that will take an OS Pet motor? The higher the motor sits the better, as that makes prop breakage on landing less likely. Or perhaps I will re-fuse the whole thing just re-use the wing. I don't really care for the profile look anyhow as there is no place to put the battery, servos etc.