Ham Shandy. Once only. Up until March 25, tourists can get the 60-day covid extension, but with the relaxation of restrictions worldwide this is likely now to to discontinued
You don't actually have a Visa On Arrival, what you have is a visa-exempt stamp. Yes you can extend a further 30 days at an immigration office for 1900 baht
He can probably get the same injection in his home country. I think it's unlikely he'd get an "unfit for travel" medical certificate. Why doesn't he simply extend his non-O visa?
There's a two day window, not 48 hours, which means if you get your test result at (say) 8.00 am on a Tuesday, you have up until 11.59 pm on the Thursday to check in for your flight. This method assists people who wait overnight for their result.
It was announced several weeks ago that people on covid extensions after having been on non-immigrant visas will no longer be eligible for covid extensions. The option was to go back to non-immigrant visa or leave the country. If your passport has already been stamped "7 days to leave" there's no option.