Second, immigration has nothing to do with DTV. It's embassies that issue the DTV visa. And no, no visa anywhere is issued based on your promise to do something.
Thousands of other people have figured out how to use the internet to contact a qualifying school to register and then apply.
every single bank branch has different requirements. Just because the branch you went to required something doesn't mean the one a km down the road will.
1) Just ask hotel for proof of your TM30 if you need it
2) assuming you have a non-O based on retirement, you need to open a bank account ASAP and transfer your 800k. You might need to get the TM30 from the hotel and go to immigration to get a residence certificate for bank account. While you're there, ask them for their handout for the 1-year retirement extension.
3) Kasikorn or SCB
4) Rental agents are specialized by area. So you need to find an area you want to live in first then find agents that know that area.
Then you were applying for the wrong visa if you needed a signature from your doctor. Only the 1-year non-OA visa needs that. What you want is the 90 day non-O visa that doesn't require insurance, a medical certificate, a background check, or any of that stuff. And no, you would not be able to switch to a retirement visa by presenting it to anyone except passport control as you enter Thailand.