Can anybody solve this hot start issue? I'm out of ideas!

On the problem at hand.....If I had to guess at this point........I'd say that the carb is jetted a touch rich in the front. I don't think the fuel is getting hot and boiling. You've done everything right to prevent that. If it was mine, I would try leaning it out on the primaries a couple of steps. Square bore carburetors tend to be fat on the primaries. Were this a 360, you might not be having this issue.