It would be definitely worthwhile having some lessons with someone that you can discuss the problem with. Personally if I were in your position (which I would be if I bought a horse now lol!) I would probably be considering 6 months worth of regular lessons before Murph was backed. I wouldn't feel I could do a recently backed youngster justice with my current length of time out of the saddle.

It's hard to give good advice without knowing your current level of wrist mobility.... if your fingers move fine and you can hold your wrist straight with forearm straight and thumb on top then I shouldn't think it would be too bad.... weight should come from the elbow/upper arm in my opinion so the wrist shouldn't really cause weight on his mouth if you are not holding it rigid, which I don't think is what happens with your wrist? The wrist should never curl, so if yours will stay fairly straight and isn't forced downwards then it shouldn't cause too much tension if that makes sense?

ETA - Can you keep your fingers soft and pliable when in a riding position?