ORIGINAL: jester_s1
I agree that everything I've read here sounds nose heavy. You're toward the front of the CG range, so move it back a 1/4 inch and see what that gets you.
On the thrust line, the relation to the cowl isn't important aside from looks. Put a level (incidence meter is more accurate and easier to use) on the horizontal stabilizer and shim it on your workbench to get it level. Then use your level to see how far off of 90 degrees your prop is compared to the stab. I would expect 2-3 degrees of down thrust and maybe 1degree of right on a model like that. If you have up thrust that would explain your diving glides (or at least contribute to them) as your tail trim would be compensating for the thrust pulling the plane up. Measure it (and keep good notes in case you need to go back) and only fix it if you truly have up thrust. Then do a thrust line test as described in the trim manual.
The plane is being pulled down... rather than up.... hence why it needs more than 1/4" up elevator trim. The climb at more than 80% power might be due to the gross amount of up elevator trim simply having more authority at greater speed and engine thrust.
We only know a few facts so far.
1. It is balanced on the prescribed CG
2. It requires gross up elevator trim for level flight at 80% power
3. It dives at low power
4. It climbs at more than 80% power
We don't know some critical facts
1. The decalage relationship (incidence numbers)
2. Engine thrust angle
3. Airfoil symmetry (symmetrical or asymmetrical)
Thrust angles have nothing to do with a plane that is diving at low power unless the elevator is trimmed down to counter powered flight lift and that is not the case... so set thrust angles aside for the moment
CG is correct... so set it aside as the cause of the low power diving.
With the elevator trimmed grossly up and it still dives at low power... that points to what you suggested.... the most likely explanation is a decalage problem, ie that the stab is too positive.