I treated the problem as a trivial "Let's get rid of the clipping!" thing, and it's working much better now. Share rates equilibrated at around 30 seconds per share after an hour or so, versus the several days at around 2 shares per hour that I was dealing with before. That will make testing and development much easier. Soon, I'll add rejection of shares with timestamps in the future to protect against difficulty manipulation DoS attempts.

I think I've found a good way to solve the replay protection thing and another issue at the same time. I'll check to make sure it will work before mentioning it. It's a little complicated of a solution, but it should be worth it for the side benefits: if I'm right, it will more fairly reward people for transaction fees while also making sure that my chain can't accidentally kill the old chain.