Do you all think it's possible that the combination of the slightly uneven horizontal stab and the slight weight imbalance could cause the right wing to drop so sharply? The imperfections seem quite minor to me, but maybe?
Maybe, but I don't think so. (how's that for a non-answer).
Did you check the wing for warps, espeically a warped aileron? It's real easy for the ailerons on some of these trainers to get a bit of a warp to them since they are just a solid chunk of balsa with heat-shink covering on them. If you heat the covering unevenly, you can twist the TE of the aileron a little, resulting in some roll tendancy.
Also, turn the radio on, and ceneter the ailerons. Now grab the aileron tip on one side and gently push up and down. Ideally, the aileron won't flex. In the real world, it will flex some. Compare the two sides, does one flex a lot more than the other? If so, it can cause some trim oddities (though, usually it just results in rolling faster one way than the other)