You can use a Y cable for the rudders but not the elevons, they need to be on seperate channels. Just what RX channel each elevon (left and right) plugs into is up to the specific radio and how it is programmed.

Yes it is elevon mixing. Elevator and Aileron function using only 2 channels on 2 surfaces. It is USUALLY mixed in the transmitter.

Do you mean are the steering and the rudder mechanically connected? Because that is pretty rare in the jets I have seen. On the other hand, the rudder and nosewheel steering servos can be ELECTRICALLY connected, but sometimes require a reversed servo as you cannot get the correct rotation given how the servos are mounted. I usually run mine this way, a reversed servo on the nosegear, "Y"'d to the rudder servo(s).

Well as I said if they are "Y"'d they are all on one channel, usually the rudder channel or channel 4. In my Stingray, I did things a little differently, I put each rudder and the nose wheel steering servo on a different channel. This lets me set the trim of the nosewheel with a knob, and also lets me cant the rudders inward together, acting like an additional speedbrake, works pretty well.

Hope that helps,