[gv550]

I did this shaft replacement repair last week on my Nomad, and paid particular attention to the timing since I was offering advice about it. This is the 4th time I've done this job. I have always set the cam timing from the balancer sprocket and this time I also tried to verify the timing after installing the alternator rotor and noticed the cam marks wouldn't align when I set the crank from the rotor marks, off about one tooth just as you say. I've never noticed this before (never looked), but since the manual says to use the balancer sprocket that's what I do. I have no explanation other than the distance the rotor mark is from the crankcase pointer leaves a lot of variation depending on what angle you look at it from.
The tensioners definitely need to be removed to reset the timing.
The tensioner is spring loaded and additional force is applied by engine oil pressure, I believe the oil pressure keeps tension on the chain when the rod is between two pawl ratchet teeth. You may notice the engine rattles a bit when first started and especially after doing an oil change, until engine oil pressure builds and applies more force to the guide to minimize that rattle. But the pawl still holds the rod so it can't retract enough to allow the chain to jump a tooth, (or the timing would jump every cold start-up).
The idea of resetting the front cam by rotating the crank 308 degrees from the R mark was yours, in post 1. After thinking about it I believe it is possible and I rolled my engine over several times to try confirm it, but you won't find anything about it in the manual because they want us to time R after setting F. It should work, but really.... after all this grief why not just pull the engine, remove both valve covers and the rotor and do the job properly. And if you do pull the engine, DO NOT reuse the tab washer!
The pulley mounting procedure that I recommend is directly from the service manual, but it's not in the maintenance schedule and definitely should be.
People need to exercise due diligence before buying anything, talk to owners, read forums, check for bulletins and recalls, etc. When I looked at this Nomad for sale I said I needed to see the output spline before I would buy it....... the spline was worn but not yet stripped and I explained politely to the seller what was involved and we agreed on a different price.