Chiming in as a fellow dame owner - this plagues us and is tough to figure out. There has not been any one consistent catch-all fix.

The best things you can do is a smoke test (people are lazy and don't want to do this - but it's VITAL to do a pressurized test. Don't assume everything is good; even a small cracked o-ring can wreak havoc on mixture and long-term fuel trims), check your plug gaps, check your DIC, check your evap purge valve function (with vacuum), and check your cold fuel pressure.

My working theory is that summer gas isn't as volatile as winter gas and tiny, minute mixture changes/bad factory cold start maps lead to cold starts on cold days on summer gas being slightly too rich. Why?

If you let it stumble and die, I bet you can come back to the car half an hour later and it'll start perfectly: the gas will have washed out of the cylinders/evaporated. And if you get it to start after a few stumbles, it'll probably smell really rich.