I may be completely talking through my hat here, but reading this thread a few questions come to mind and they just keep on nagging me.

1. Did you have a similar issue with the Snipe? The SE5a SHOULD be a much more stable platform and much easier to fly than the Snipe. It's much longer coupled after all and has more dihedral...
2. I have seen Bipes with MUCH less care taken with setup (in the wings at least) fly just fine. A friends Mick Reeves Pup for instance has a massive warp in the lower port wing and he is able to fly it quite adequately with only a trim change.

The idea that keeps nagging me is that it MUST surely be Engine thrust angle that is the issue.

Cheers,

Hugh