If you're talking about a foreign card, you can take 30k at a time from the ATMs of Krungsri, TTB (formerly TMB), and Citibank. CIMB also allowed this, but I think they've gotten rid of all their machines, other than perhaps a few attached to physical branches.
I'm not aware that the location of your bank branch would make any difference to immigration - I've heard of many people who have continued to use accounts at branches in other parts of Thailand after they moved, simply because it was too much of a hassle to change. (From what I understand, no Thai bank will let you just change the location of your account - you have to close the existing account and open a new one at the other branch.)
Unless Phuket is using non-standard rules, or the rules are changed before you get here, you would not need to show health insurance to convert to a non-O (retirement) - insurance is only required for OA and OX.
You have to have at least 15 days remaining in your authorized stay when you apply for the non-O. They will then tell you to come back in two weeks for the "decision", but I've never heard of anyone being denied at this stage - what it means in reality is that you come back to get the visa and new period of stay stamped in your passport. (I don't know about Phuket specifically - I'm only talking about the rules from headquarters.)
If you're applying for a non-O in Thailand as a "conversion" from a visa exempt or tourist visa entry, the 800k only has to be in the bank on the day of application - the two month requirement is only for extensions. Since you'll be given a three-month stay when you receive the non-O, there will be plenty of time to meet the seasoning requirement by the time of your first extension application. Please note, though, that the in-country non-O application requires you to show that the 800k came from overseas, which is not required for extensions.
Foreigners are currently banned from transiting through Taiwan. The flights booked by the OP may still be operating as scheduled, but EVA won't be allowed to carry him.
There was nothing in the "Remarks" section at the bottom of the page? The one time that I got the "rejected" result, they explained what they were unhappy about in that area.
A friend of mine used a letter like that a few months ago and it was accepted, but it probably depends on which embassy/consulate you're dealing with, and perhaps even the individual officer who adjudicates your application.
The requirement for the US is three days (not 72 hours) prior to departure from the point of origin, and the three-day period applies to when the sample was taken, not when the result came back. From the CDC:
"'Why does the Order specify 3 days rather than 72 hours? What is considered 3 days?'
The 3-day period is the 3 days before the flight’s departure. The Order uses a 3-day timeframe instead of 72 hours to provide more flexibility to the traveler. By using a 3-day window, test validity does not depend on the time of the flight or the time of day that the test was administered.
For example, if a passenger’s flight is at 1pm on a Friday, the passenger could board with a negative test that was taken any time on the prior Tuesday or after."