It was the four-screw piece that sheared off the three-bolt black piece. They were over-tightened and/or way worn out, obviously. I was riding 300+ miles every week getting ready for that double century.

Cleat position may have been off enough to cause the hot spots, though you could make the argument that it was pedaling for ten hours that was the real cause.

The Shimano pedals and cleats are bigger which spreads the pressure over a bigger surface. They have been fine so far. I'll find out at Alta Alpina in June exactly how fine.