Newbie,
Mechanical retracts can be a PITA. I would save those for your first scale job. Secondly, your mechanical retracts usually need a 180 degree servo. One servo will operate both retract units. These servos are non-proportional, and once commanded, move the full 180 degrees until they stop. Some people have used hi-torque normal servos successfully, so I know it can be done.

I think a better solution would be to add flaps with the extra channel. By the way, I enjoy your enthusiasm for this sport. You remind me of myself when I first started. Keep it up!!