they need to provide a letter. i don't think they would do anything beyond that officially but he could probably convince one of the nurses or admin staff to visit immigration for him on their day off for much less than the agency quoted him.
you don't need to pay an agency for a medical extension unless you are so ill you can't go to immigration and don't have any friends/family willing to go on your behalf
there wont be any decent vaccine readily available around the world anytime soon. the thai government will just continue slowly approving new hotels to increase quarantine capacity and gradually easing restrictions on who can come to fill the space.
yes even residents, semi-residents, and regular long-stayers, who are currently outside thailand are enough in number to fill the current capacity of quarantine hotel for quite some time. they would have to approve hundreds more hotels which will take years at the current rate, before they have to start worrying about how to attract regular tourists to fill the space.
most aren't but plenty are. remember pre-covid thailand had over 3 million arrivals per month. even 1% of that is over 30,000, which is already more than the current quarantine hotel capacity.
most of the quarantine hotels are already full or nearly full, even though only people with very specific circumstances are currently allowed to apply for a certificate of entry. this scheme will expand the number of people who can come much faster than the government can approve hotels as quarantine facilities. obviously the majority of thailand's normal tourists wont come but only 1% would already more than fill all the quarantine hotels.
if you work here you can get a visa for that. if you study here you can get a visa for that. if you are over 50 and retired you can get a visa for that. if you are so rich you have taken early retirement below 50, you don't need to worry about "affordable".
the latest i heard on the plan for the scheme for phuket (plus other locations to be confirmed) was very similar to this (basically anyone from japan, korea, and also the uk, denmark, germany, and hong kong) can come if they manage to get a flight (and book a return flight more than 30 days later) but either way the numbers are tiny, because of the limited quarantine capacity (i guess that's the bottleneck in vietnam too).
that's only around 20,000 people per month, if that. similar scale to thailand's quarantine scheme. pre-coronophobia, vietnam had 1,500,000 international arrivals per month.