Thank you for all of the comments and insights.
We have gained several inches of pond depth since I started pumping creek water in on Sunday. I do not believe we lost any more fish yesterday or today, and the pond is noticeably cooler now. I am aerating 24/7 again.
One thing I have learned on this forum is that there are lots of different opinions. I'll never know the exact cause or whether there were simply too many pounds of fish to survive a sudden heatwave. One particularly puzzling aspect is that we never observed any fish gulping for air, as I would have expected in an oxygen crisis.
There is another possible factor that I forgot to mention. After the 16 oz. Fluridone treatment and a week or so before the heatwave, we were overrun with a flock of Canada geese for several days until I had a day off from work and was able to encourage them to move on. They crapped all over the pond and it's banks. That dose of "nutrients" plus the decay of DW certainly didn't help.
