OK...

Here some aerodynamics for you:

If your plane tends to pitch up during a high speed dive, this usually means you have too much incidence on the wing (or tail, whatever... but something is off).
Moving the CG backwards will not help thing... it will just make the handling worse!
What you discribed sounds like a too much incidence... it could also be an aft CG... but if it was that twitchy to begin with, and you moved the CG even more back, it would have become hardly (or at all) flyable. So it's probably the incidence.

The roll control has very little to do with the CG ot the incidences... there can be some influence, but not like you stated.

Adjusting the incidence is a piece of cake. Just shim the wing some, and try it out. When you have found the right setting, fill the gap you have between the fuse and the wing with epoxy and micro ballons.

Happy flying...