@ balsa or carbon:
'....It looks like Flattermann's solution #7 above ( universal joint solution ) would use an elevon mix . That seems to me to be a good design because you would have TWO servos pulling the weight of the motor and prop up when the input is up elevator. ...'
No use of an elevon mix - simply up/down, right/left. The design of a Universal Joint prevents twisting in unwanted directions.
Look again to Pic. # 7 ...
As I reported in another thread, this solution does'nt allow as much steering angle in all directions as the # 6 allows, because the right/left and up/down tuning axles of a universal joint are not on the same turning level - they are one behind the other.