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.
I've always been unclear if it's 3 months after the extension is issued, or if it's 3 months after the date of the extension. I always say date of the extension to be safe. But if they take it 3 months from the date the extension is issued, you might be okay.