3. I have no ideas about waiting times however the SM and General roles are vastly different and you missed out the possiblility of MA(RM)!
I did do a little research on this but I read that the Royal Marines didn't train MA's anymore and once you were trained as a Navy MA you could attempt a easier/watered down version of the All Arms Commando Course and then serve with the RM's. But then again I read this on a forum like The student Room or Yahoo Answers so I don't know how reliable that is.
General Service MA's spent time in the hospitals (Haslar / Stonehouse) and manned Sick Bay's ashore and afloat. Large ships (Carriers / LSG's etc) had a full Medical Team eg Dr, CPOMA, POMA, LMA's and MA's while the small ships (Frigates/Destroyers) had an LMA or POMA and unless you were on a big deployment you were on your own ... no Dr - therefore a lot of responsibility - the buck stopped with you.
SM MA's in my day were only on the nukes as O Boats were still running and covered the Sick Bay as well as the Nuclear Physics side of things.
Thanks for the insight MG Maniac, this is exactly what I was looking for MG Maniac. I'll have to do a bit more research on which role may suit me more (if I'm lucky enough to get that far).