The DC embassy was processing pre-approval and final approval in 1 day each last month. They do priority based on flight date. You will probably be fine.
I have never heard of a "false positive".. Just because you test positive once, you have minimum 2 week hospital and need 2 negative results and then 2 weeks home surveillance. I don't agree with it, but those are rules.
I am a former stv... You can double check with embassy directly.
I did know one person that did get in and renewed using safety wing from Europe.. But they were in province for extention and immagration allowed it.
In Bangkok they would not allow extention without Thai insurance per the immagration officer that did my extension.
I recommend it either way, as most nonthai insurance companies will no pay for asymptotiatic hospital stay required by Thai government if you test positive. Many foreigner left with big hospital bills their insurance won't pay. Thai insurance will cover it.
as far entry stuff due to covid, it doesn't matter the visa or how long you are gone. You will need to quarantine or participate in the sandbox. Covid tests, insurance, blah blah blah.
Prior Oct 1, you had to be in a low or medium risk level country for 21 days to be allowed in sandbox... Which excludes thailand. The rules changed a bit October 1..but check if you can participate if you only left for only 7 to 14 days.
AXA does allow you to extend your insurance once for 90 days. So if you are planning on staying the 270 days, best to buy all up front instead of only 90 days.
The first question on the COE is visa type... You can select visa exempt if it applies to your passport and then enter thailand with the TR60...
However, that does not make a lot of sense as getting a COE only takes a few day, and a visa is longer. So you probably doing your COE too far in advanced.