All clearances should have been checked prior to build even with second hand parts!! And then resized to suit, even if the rod was to blame it should have been noticed on build as being out of spec, not just lobbed in with new bearings!
This is my point too it's basics that seem to have been overlooked or they have baffled you with science as to what the real failure was
He might have covered 500 miles without issue but this was the breaking in mileage so careful driving.
Hope you get it sorted, ultimately on a fresh build they are liable unless you have dumped the oil and took for a drive its not like a map where they can argue this that and the other, and unforeseen circumstances, they have built an engine and it should work, you may have supplied a part and if it was within tolerance there would be no issue, so if they have checked it and put it in then it wasn't to blame for it spinning. There is another issue I would say,