I have set up elevons with triple rates on Switch G. At high throws the elevator and aileron travel are set at 100%, the way I want them, but these seem to be additive if I do a full up (or down) and full roll stick movement at the same time, driving the surfaces well past the 100 percent position. Is there any way to limit the overall elevon travel to no more than the 100% throw distance at high rates with both full elevator AND aileron action, yet still get 100% elevator OR aileron movement at high rates when move separately?

There isn't a way to limit total throw n the 7C transmitter. Only the 12FG, 12Z, and 14MZ have settings for the total throw allowed for each channel. Those transmitters have settings for the normal end-to-end throw of the stick, switch, or slider, and then a total throw allowed when mixers are involved.

You'll have to set up the controls so that when you have the mixer to the extremes, the two channels mixed don't over drive.