Flying Pay:
These are the RN rules but I bet they are the same for the Army/RAF. If you are on higher rate, when you PVR you go down to middle rate (for the time you serve till you leave). If you are on middle rate, when you PVR you go down to lower rate etc etc.
FRI:
You have to pay it all back I'm afraid (then claim the tax back later) - by signing to accept it you signed to be bound by this rule.
HTH