The shims have little to do with pulley/belt alignment, those are for proper s/c clutch engagement.

Is it safe to assume you have verified you are running the belt around the pulleys the right way? Almost sounds like the belt is riding with the flat side against a ribbed pulley.

I haven't heard of a stock crank pulley grenading, but doesn't mean that it's not possible.
I do however, remember reading a thread where the person installed their belt one groove off and it was shredding belts. But to be off 4 times in a row would be unlikely odds.