I heard a power increase just before it dove. If it was approaching at low power, it was trimmed up too much to produce a good low power glide slope. That fits with your description that at half power, it seemed tail heavy.

That leads me to believe it may not have been in good low power trim, therefore I stick with a position that an incidence problem may exist.

The first effort should be to check the balance. Then trim the plane for a proper low power glide slope. After which, take note of the elevator trim. If it is trimmed up, then in very small increments adjust the wing incidence up and if trimmed down, adjust the incidence down... but I'm betting it (elevator) is currently trimmed up. If so, then adjust the incidence up in small amounts till the elevator and stab are neutral with each other and a good low power glide slope exist.

Once the low power glide slope is good with a fairly neutral elevator/stab relationship, then it is time to correct high power pitch trim with thrust changes.

In most cases of asymmetrical wings, down thrust is needed to counter increasing lift of the wing at higher speeds but as Rodney pointed out, if the engine is high up, there is a couple between it and drag center that can affect the pitch and therefore the motor thrust setting. That thrust line will be a balance between pitch down of the engine drag couple and lift generated by the asymmetrical wing so it is anyone's guess to what thrust angle is needed.