All online. My Thai partner got his within minutes of submitting the final documents (I believe non-thais may take a little longer). Once approved, just print it out. Personally, I'd print 3 or 4 copies "just in case"
The sofitel by heathrow terminal 5 does overnight PCR testing. You can get a LAMP test at the airport in 1-2 hours (though few destinations accept these at the moment)
If yoi mean that you're taking 2 flights (for example London Doha Bangkok) and the 1st flight is late for reasons outside your control, arriving after your 2nd flight should have left - then your airline puts you on another later flight, I would think that your original booking plus new onward flight should do it as it was outside your control ... though you might be delayed explaining it.
It must be at the first place where you enter the kingdom. Arriving bkk? Then bangkok (or pattaya as it's very close to bkk). Arriving HKT? then phuket it is. No domestic flight or journey permitted beyond the direct trip to the approved hotel.
i agree that Pattaya is possible from bkk because of the proximity. But you could not make a trip to quarantine wherever you like (eg. No domestic flights)