gg3 - Sounds like your issue will be fixed with the new relay. That would be good news!
welded the bendix to bendix shaft, pinion (part 16) rotates freely but I couldn't get it out to reweld the shaft.
If you have trouble in the future that pinion shaft might be something to check. I got mine out by applying heat (from a heat gun), then tapping on a hard surface.
You've probably seen the following video (from MotoCenter, Mesa AZ).
https://www.youtube.com/watch?v=CXXjFw9ghDE
My bike sounds just like that - click, thunk, tries to turn over but can't. I did weld the pinion gear to the shaft but that didn't solve the problem for me.
It does leave me with the theory that something in the starter drive system binds when power & torque is applied to it. Like your bike, without the spark plug installed it turns over just fine!
I'm going to see if I can find someone with a good starting bike who will swap starter cases with me for a minute and see if that isolates the problem.
Jeff