That's not a requirement for a retirement visa, it's a requirement for a retirement extension. If you are on a retirement extension and your bank account drops below 400,000, technically your extension is invalid as of that moment. You could theoretically be charged for overstay from that day forward. I've never seen that actually happen though, but what will happen is when you go for your next extension and they ask to see the 12 month bank statement, they will see you failed to meet the requirements of the previous extension and deny your next one. You will have to leave Thailand and start over with a new 90 day non-O visa.
For an METV, many embassies want to see a ticket into Thailand, a ticket out within 60 days, and another ticket into Thailand. This supports your need for "multiple" entries. I've also seen reports of embassies ALSO wanting a return ticket back to your home country at the end of all that.
It's not a tourist visa, it's an education visa. Whether the bank will let you open an account or not is up to them. Many reports of people with ED having issues.
The official rules are that you're taxed on money remitted to Thailand if you're a tax resident. Income sourced outside of Thailand doesn't mean you sent it to Thailand. It's much more complicated with dual taxation treaties, but money earned does not equal money remitted.
I've also heard reports that some people see the correct code on the Thai language statement but not on the English statement for some banks, as odd as that sounds.