The signal will stay the same. Use the channels you need and whatever ports are left plug the power leads to them. It does not matter which rx port you use just not the original battery port on the rx.
You can do this but I find it confusing. I see no reason you should not use channel 1 or 10 as your throttle (on a JR/Spektrum receiver), depending on which lines up with throttle on your receiver.
The 5V is just powering the receiver. This has nothing to do with the servo power. The servo power is directed straight from the battery inputs to the servo power pins. The servos have no idea what voltage the receiver is running on and they don't care.
Can you tell me please which particualar ports on my JR 921X reciever, I should plug channel 1 and 10 from the Powerexpander 10 into. Do both get plugged in?
You can plug the channels from the Eq10 into any servo output ports on your receiver. DO NOT plug any of the Eq10 connections into any battery/bind/data ports. You can plug the Eq10 channels into the receiver so your servo wires come off the Eq10 on the side that makes routing them the cleanest if you like.