Covid insurance is like $40 bucks for 30 days. Don't be a cheap charlie and try to work the system. Buy the damn insurance for the time you'll be there.
layovers are not restricted. You are not entering the country outside the airport, so you can layover anywhere. Airlines would not be able to fly through if you couldn't.
My personal opinion. I think COE and at least 3 day quarantine is going to be the norm. Reason I think this is 1st money. 2nd 3day quarantine allows them to PCR test people arriving off flights. Just my thoughts.
Sadly you might be denied. Had a friend get denied 1hr over coming from US on JAL airlines. It's not Thailand who denies you, it's the airlines. I would call their customer service ASAP. Also if you have time I would call a site that does the test and pay whatever their ridiculous price is to get results ASAP. They can literally give you results in 20 min if they wanted to.