Silly me, I thought the pin went through the extension into the barrel so it couldn't rotate out.
http://www.ar15barrels.com/making.shtml Check the third pic from the bottom.
Barrel nuts will loosen, it's the whole point of the star cuts - to stop rotation by snagging the gas tube. I doubt Stoner would go to the trouble of doing that and ignore pinning the barrel extension. If it unscrews, you lose headspace and catastrophe is imminent.
As for shims, just lap the upper, it not only squares it, the nut will rotate to line up without having to stack shims - and have it still out of square. Just Go Slow and Check Often. It doesn't take much.