I think your fantisizing a problem where one doesn't exist. Solid bulkheads work fine with no stress problems whatsoever. I personally have built 7 wooden kayks (5 strippers) all with solid bulkheads. I have loads of friends with solid bulkheads in strippers and S&G's. Nerver even heard of a problem.

This stress riser thing migh be a story from someone's poorly made glass boat?? I don't really know. The solid bulkhead is going to give your kayak great strength and rigidity. I'll bet about about 95% of the wood kayak world uses solid bulkheads. 5% foam for those who don't want to put the installation work in.

As far as your wood kayak being flexable, it's not. It's rigid and is not going to bend over rocks.