Hi Quince
I like the set up, the lack of a gearbox is good with the belt drive takes out all the alignment problem. I've worked with VP props - and Kitchener gear - and believe that, properly maintained they are as reliable as a gearbox.
You could get away with fixed rpm, imho, but I agree with most of the reasons above for having a throttle, and also that warming up on tick over or leaving the engine running to heat domestic water / charge the service batteries would be far more comfortable.
An outfit I've worked for before had a V8 GMC in a work boat, it was reputed to have come out of a small tank and went from tick over to flat out with very little adjustment in between, consequently it was hard on gearboxes when shifting from ahead to astern, something a work boat does a lot of...
With the revs fixed you may find the same wear issues on the VP prop. If you could link the pitch control to the throttle to drop the revs as you passed thro' the neutral point it would save all sorts of wear on the thrust bearings etc. and give you a smoother input of power to the prop.
It's not an engine I'm familiar with, doubtless others here will help, but given the numbers of the LP motors out there I don't think there would be too much difficulty with the throttle and I'd hope that a variable speed governor would be an easy swap.
Looks like an interesting project, keep posting as it develops!
Cheers
Stef