Ive seen crank cases where the crank has been fitted wrong and has done crankcase damage.
Like you said, once the bearing is in place and the cases bolted up that bearing isnt going far. I would doubt even a seized bearing would shear its pin. I guess the crank has been incorectley fitted.
Do the cases show any signs of hammering (looks like black marks) where the bearing has been loose in the case? If thats the case then all the load will of been taken on the pin.
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If I have to take the carbs off once more...