a couple of top questions?

You can bet a few of those bolts have been left out by others. Only ones that are critical position are the ends.
If I was going to the trouble to get the broken piece of bolt out, I would buy 10 new stainless bolts and a stick of aluminum ( easier to drill than stainless ) to make the wing washers.
FYI factory used a lead/zinc coating on a lot of their steel hardware similar to what is used on chain link fencing.