RE: dual rate offset?
The most common reason for servos to move centre when you switch dual rates is that the pot on the transmitter stick is worn or not properly centred, so that the radio thinks the stick is being held some distance away from neutral when it isn't so of course when you switch rates it reduces the travel it thinks you are calling for. Call your service agent, it may be that a slight tweak of a trimming pot will electronically centre the stick again, if not return to the service centre for it to be sorted.
H