Andrew,

Since you mentioned angle of incidence this may be a good place to mention that you may want to look at the decalage. In your case, decalage is the differences in angle of incidence between your wing and horizontal stab. It is also the differences in top and bottom wings for biplanes.

This may sound like nit picking, but it is a major point with some models.

Just a personal opinion, but I avoid down thrust on engines until all else fails. You are compensating one problem with another set of problems.

Is there any way for you to shim the trailing edge of the wing? I've done that with soft pads, and sometimes with ply glued in place. If I need to shim I always take various thicknesses out to the field to experiment. The plane will probably track and groove better if you get the decalage set better.