I am not quite sure about the working of the complete assembly in all details, but there are some reports about loose nuts
. Obviously that can happen even after torquing them down a lot. I guess in these cases the not so perfect fit within the carrier/gearshaft spline joint was a very sloppy push job. So even if you do not look at shocks from the chain you will agree that any play, say two thou in the spline , will produce an orbiting of the carrier around the gear shaft of a 2 thou circle at each revolution of the clutch assembly simply by the chain pull. Whenever you do up the C 20 nut it will load all components onto the PD 21 spacer and ball bearing. BUT between all components there are places that will not do that 2 thou orbit like the carrier on the spline so there will be wear at that spot and soon there is no more nip in the stack of parts. That may explain why some nuts stay put, some not so. I would not dare to trust into the amount of nip in this place plus the safety wiring through the nut and shaft but apply some very mild Loctite on the thread like 222 . That is like a nyloc nut effect, you do not want a stronger type on all of the fine, big thread, you´d be very challenged to undo the nut later.
Basically there should not be a lot of a twisting action on the C 20 nut apart from loosing the preload due to axial wear in places. But I suspect there may be some negative input from the bronce bush in the C 13 clutch carrier. How is that bush lubricated , hardly ever ? So when the setting of that servo system is somewhat uneven between the two shoes I could imagine some very high sideload on that bush cum C 20 nut , a lot of friction in consequence and possibly loosening the nut from side loads. Certainly I cannot prove that point , just adding to the discussion, no practical experience on Vincents so far.
Anyway, whenever there is some wear within the spline any heavy tightening of the C 20 nut will not be a permanent fixture, so you might as well just screw up finger tight and add some other safety means like Loctite on the thread. I would not trust the safety wire alone.
Vic