bad forward O2 sensor will cause you to use more gas but should not cause the starting problems you're describing. I'm leaning toward a wiring issue or connectivity issue with your coolant temp sensor at this point. Electronic issues are a real pain when they're intermittent like this. Might as well replace it though and clear that issue up. They don't cost much, I believe yours is one of the single wire variants.
If your metal retainer is on the clip, there shouldn't be an issue at the actual connection. I don't know what else to try at this point but to get the car warmed up and see if you can get the problem to reappear and mess with the harness at the cts and around the front of the valve cover like before and get it to clear.
if you get it narrowed down to a bad harness side connection, Wiring specialties does sell individual pieces too. https://www.wiringspecialties.com/ka24de2.html
Hopefully someone else will chime in with an idea. Once you are able to reproduce the problem I'd check for spark on all cylinders when it won't start. You can do that pretty easily with a timing light on each of the plug wires in sequence.
What's the situation with the AC fan? does it come on when this problem is happening?