S.O.S.,input and ideas please!

I think you'd still be able to work a bike throttle with finger and thumb action. A thumb throttle could be set up but might not have fine enough control. I have a neighbor who has a fused wrist, layed a tile floor in my house with me helping but I didn't notice his wrist action until a couple days into the job. It didn't affect him much. His wrist was shattered when working on a fire truck engine when someone shut the hood while he had his hand in the engine compartment back near the hinge. One thing that will help is the Goldwing's cruise control. It should be possible to reverse the throttle and clutch so you had a throttle and brake on the left with the clutch on the right. The brake and clutch shouldn't be hard to reverse mainly it's just the hoses and rewiring the switches. With a bit of ingenuity the throttle should be doable. After that it would just be the matter of getting used to it. A bit of practice should handle that. It wouldn't matter too much which switches were on which side.
One othe possibility I can see would be using a throttle boss which should give a pretty good range of motion using your fingers and the heel of your hand.

I too have ridden with my arm in a cast -- not as aggressively as without, certainly.

I remember flapping my elbo up and down to control throttoel and needing to strengthen my left arm for some of the slow speed manuvers.

Most anything that can be said, even by someone that has gone through the procedure, would be subjective and may not apply to your situation. Therefore, I would suggest experimentation and research. If you're about ready for fusion, doubtless you have a wrist brace of sorts available to you... Barring that, you've probably got an old sock, a ruler (or a few sticks) and some duct-tape...

Why not try to imobilize your wrist joint as you physician has described? Splint across the wrist to preclude yaw/tilt/roll and try to figure out how you'd feel. Fully knowing how this could work for you will probably also help your mind be at ease with the procedure and more accepting of the outcome... Why not just try it?