RE: nose dive first test - zagi
There are few things to check. Of course the CG is very important. This should be according to the plans. The elevons should be set at the correct angle. When viewed from the side the elevons should be approx 15 degrees above neutral. This should give you enough reflex to fly straight and level. The motor trust line should be inline with the flight path. Basically parrallel with a line drawn chordwise and - measured from the middle of the nose through to the middle of the trailing edge.
My guess is that you have not enough up elevator trimmed into the Zagi. Does that sound like a possibility to you?