Small problems:
1: Who would we frame you to? Besides you, no one is playing the king's side (sadly) so unless you want either of us to play a commanding officer this creates a problem.
2: How would Rikard frame you? He has not infiltrated any high command or planted incriminating evidence. I could pretend another Frumentarii HAS infiltrated, but then how would he/she know what has transpired on the road.
3: It would be very out of character for Rikard or even the prince to not only come to your aid, but also trust since they were fighting or opposing you respectively several days earlier.
4: Rikard is insane to the point of psychotic behavior; he will not stop attacking until either of them are either dead or dying. That's his whole shtick.
I understand why you'd want to switch sides, there are after all more players by the prince, but your suggestion just has too many plotholes. How does this sound: Rikard defeats you and drags you back with him alive. If you agree, your position will be less than enjoyable as you will be a prisoner of a psychopath.