Moreso than the wing on it's own it is the angle that you have between the wing and the stabilizer that produces the nose up reaction to speed. But linked to that is the location of the balance point. Or Center of Gravity (CG) a many of us also call the balance point.

The trim angle needed between the wing and stabilizer is linked to the balance point location. If the BP is well forward you need a greater angle. Further back and you need less angle. Too far back and the model won't want to recover from a dive on it's own.

The trim is somewhat variable ONLY because we typically use elevator trim to slightly alter the trim angle. Note that with a model such as your trainer you can only adjust the elevator trim for level flight at one power setting. Or more accurately you're setting the level flight SPEED. The speed coming from the power setting. Lower the power and the model will go into a glide at nearly the same trim speed. Raise the power above the level flight pointand the model will react by climbing at an angle that maintains the trimmed airspeed.

By moving the BP forward like your one expert did he moved the trim speed to a slightly higher setting. But your other expert that suggested foam under the trailing edge did not know that you had already put in the nose weight. The forward BP from the added noseweight and the lesser wing incidence angle from the foam will be working at crossed purposes. To finish the foam under the wing mod the proper second step is to remove the added nose weight. If you don't then all that will occur is that you'll add enough elevator trim to cancel out the reduced wing incidence angle.

If you're at the point where you are flying well enough that you are finding that the model's desire to "balloon", or lift the nose with added airspeed, is getting in your way then the proper modification is to trim theBPBACK about 3/8 to 1/2 inch. Then work on shimming the wing until you don't have a lot of down trim in the elevator. It is the BP that drives the need for a matching wing to taildifference angle that affects the flying. The wing to tail angle needed is purely a product of the BP locationand it is the BP location that sets how strongly the nose up pitch in response to power or diving speed will be. Thewing to tail trim anglethat actually does thepitching actionis slaved to theBP location by your desire for the model to fly level at some cruise airspeed.

Does that sort of help to tie things together for you? I've attached a simple all sheet balsa glider plan. If you build one you can play with a lot of the features I've discussed. You can also alter the BPback and forth and then play with the elevator trim to see how as you move the BPback at some point the model will no longer recover from a stall or a dive. Then as you move the BPforward in 1/8 inch increments and re-trim the elevator for a slow and stableglide how it becomes marginally stable with the first move, and more stable with each additional move. At the same time try a HARDstraight forward throw with each setting. Note now at first it does a gentle rise of the nose and a long upward curved path before falling off in a stall. But as the BPgets two or three more steps ahead and the elevator is re-trimmed to compensate how a hard launch produces a tight loop that tries to hit you in the back of the head. This is the result of the link between the BPand the wing tostablilizer trim angle forstable level flying speed that I've been talking about. If you understand how this all works then YOUwill be better than most of the experts at your field.

If you joined the wing center joint together with some angle difference there really isn't much you can do to compensate for that which will work over the whole speed range. You can try to warp the wings so that they compensate but in the end the best option is to cut the center joint apart and re-do it with the two sides dead even. Depending on your construction abilities at this point you may find it easier to just use the plans from the kit and build a new wing. If you go this route consider it as a first step into scratch building. If you want to try it then you'll get no lack of help from here for advice on how to do each step.