Can I suggest you get this book
http://gb.trapletshop.com/simply-model-submarines-by-chris-jacksonI did and it helped me and it is not as advanced or technologically challenging as the Submarine technology one

Also I wouldn't mess about rebuilding various radios into 1unit it would be easier to go and invest in a F14/16 radio
unit if you can then look at the add on boards for what you are needing or wanting to do.
anyway to answer your questions.....
What I use on my Trafalgar is a standard Hitec 6ch unit
Stick 1: speed control
Stick 2: Rudder and Rear planes
rotary switch: ballast tank
I have not fitted the front dive planes.
For my S Class it is my intention to use a F14 navy.
stick1: speed control (independent)
Stick2: rudder and Front planes
secondary switch(S) TBC: ballast tanks
Rear plane working off a sub level unit
I would stay clear of the mixers for the planes, unless it is a X-tail configured submarine
toms suggestion for a mixer for speed control is worth considering
there is a few submarine weekends being planned they are posted on mayhem you may want to go
visit these and take your submarine and see how the experts are doing it
