RE: How much pitch control would elevons provide
That bit above about flap spanwise position might need explanation.
Most models are like my JayTee and have the flaps inboard ahead of the tail. When flaps go down I need downtrim to hold the nose down.
However some model jets have twin booms, and the flap position varies. On the Baby Boomerang the flap is between the booms, right ahead of the tail. It needs down trim with down flap.
However on the BVM KingCat the flaps are placed outboard of the booms (so they are outboard of the tail as well). It needs UP trim with down flap. Some owners use up aileron (crow) to help counteract the flaps' nose down pitch.
On the Falcon 120 the flaps are partly inboard of the boom and partly outboard. The result is a negligible pitch effect from flaps. No trim change.