I have a 7C-2.4 GHz transmitter, and I have a mixing problem. I'm using two aileron servoes and two elevator servoes, and I have activated ailevator and flaperon like the manual describe. Trying knife-edge (KE) my plane rolle back to "upright"-position, and I have to use 2 of the 3 free mixes to get rid of the problem (one mix to for each of the aileron servoes). When that problem was solved my plane pitches a bit to "the bottom" side in KE, and I want to mix that tendency away, but now I only have one free mix left, and I need two mixes (one for each elevator servo).

I wonder if there is any way to use the free mixes (or the preprogrammed mixes) to solve this problem, or if there is anything in the manual that I have missed?

You've missed nothing. The 7C just can't do all of the mixing you want it to do. You need more functions than the 7C is able to provide.