Servo extensions used in my plane are as follows: 2 18" for Elevator, 2 12" for wings, 2 6" comming off reciever aileron to allow easy plugging into wings. I run a single 2000mah battery on the reciever and a 2000mah on the ingnition. Both batteries are Li Ion and have been regulated down to 6V. I used some sullivan gold rods to move my throttle and choke servos back into the plane near the CG. This kept the nose light, kept my radio gear away from the engine ignition and eliminated the need for more servo extentions. It's very easy to do and I recommend it.

Only one glue is needed for these planes, Gorilla Glue. You use it for installing the hinges. Once in a while a glue joint might loosen on a friends plane and having some medium CA in the field box is always a good idea.

FWIW, depending on your engine you can get away with a 4.8 V 1200mah Nicad and be just fine. Any good 6V 1800mah NiMh battery would work on your flight controls. These would give you an average of 4 flights on a single charge. Now that you are into the "big birds" I would buy better power though. But this will get you by in a pinch.

Dual recievers work when you split the flight surfaces. When a reciever goes bad the dead surfaces go to neutral while the other surface are still operational. You have to have radio gear that has fail safe capability. This way you can still land the plane. Running dual antenas out of this size plane is a little tougher. There just is not as much room for to antenas and keeping them from crossing. FWIW, I run a single reciever PCM reciever. I think dual recievers for a 33% and bigger is good. But these planes are just not required.

Sorry, I can't help on the Zenoha question.