Hi Richard and well done your Dad for making it home in such terrible times.
Probably wise to start a new topic, however much we try to compress it!
I guess it's possible that a slack cush drive nut could make an end-float problem worse. I am just a makee-learnee born again A10 person - there are many here who really obviously do know stuff and i hope some of them will chime in. But the idea that crank end-float is controlled by the drive sprocket and cush drive nut - - sounds all wrong to me. Half a turn of a nut with say 18 or even 20 tpi is so much more than 5 thou, if you think about it . . . we'd be running around with mega- or minus- clearances were it true, and be none the wiser cos you can't measure it properly. No, the clearance is set, on the bench, using shims inside the cases, with a dial gauge to measure with, and often a fair bit of trial and error assembly and disassembly!
I really don't think the sliver of crankshaft shim you found in the sump plate came to be there just because the shock absorber nut wasn't done up fully. You shouldn't be able to move the crank more than the prescribed (small) amount whether the engine sprocket and associated parts are attached or not. If the nut was loose, it's possible I guess that any excess play present inside the crankcase would be magnified by the extra weight of moving parts thrashing in and out on the end of the drive side main-shaft like a slide-hammer, but I don't personally think anything outboard of the crankcase caused your big end to go. That was due to lack of oil - flow, pressure, or both. I don't believe (until it seized!) that the oil holes in the big end shells suddenly got blocked through mysterious misalignment. Maybe it's because I'm just too ignorant to know what I don't know, but I'd still suspect wear or some other defect in the timing side main bush for a lack of oil pressure. Or the pump. Or a blockage. I see that some people have reported that some types of main bush, constructed of an inner and outer sleeve of different materials pegged together, could possibly fail as a result of one part contra-rotating against the other and misaligning the oil feed holes between themselves and the timing side crankcase oil feed hole. I don't know. Others say that the two-part sort is better than a pure phosphor-bronze bush, which may wear the crank more quickly. I'm not a metallurgist and have no idea. Whichever sort is used, the holes feeding oil from the pump to the bush through the casting need to be aligned with the holes in the bush, obviously . . . if the bush turns in its housing cos it's a poor fit . . . result: oil flow reduced or cut off, and first casualty, lh big end.
As to the pump, I'm not sure what to say. If oil returns when running , that's a good start and proves a) the pump turns at least some of the time and b) the anti-syphon valve isn't stuck shut. Oh, and c) there's oil down there. The only way I know to verify the thing is OK is to take it off, take it apart, look at it, and look very very carefully at the worm drive that turns it - the driving as well as the driven part. Are the teeth good on the internal gears in the pump, especially the feed side? Is the meshing of the worm gears good - no tight or loose spots? Are there chips round the edges of the 'teeth' cut in the worm? If in doubt, although expensive, a new pump and drive pinion is a helluva lot less than a(nother) rebuild! If you truly had far too much end-float on the crank, this is NOT a good thing for the worm gear which has to accommodate it somehow, as it doesn't have its own private cush drive built in.
Whatever, if you've cleaned everything, checked everything, replaced everything necessary, the good news is the thing will run fine and all this will just be history. Let's hope people who know more chip in with wiser words, which I'll look forward to reading. Groily
PS Also check and clean oil lines and oil tank pipework for clear delivery flow!