RE: Diamond dust cg check
IF you want to continue to try using that motor you can move it back a good 1/2 inch maybe more. Also you could side mount the servos and move them back some. Side mount the throttle servo between the two elevon servos.... heavier battery since you'll need the weight you might as well get benefit...
But even with all this she's going to be tough to balance, shame it should be quick with that motor.
Second choice is to get a Magnum w/o motor for the Jett and get a lighter motor for the DD....