You'll need the tm30. Explain to immigration along with all your paperwork. If that doesn't work you'll need to contact the police and look for a new place to live.
you asked about legal protection. As a forigner, it's very doubtful you will receive any protection. Thus assume the money is a loss. If you can't afford the loss then I would not do it.
O-A visa good for one year. Easiest to obtain while in your home country, as the requirements are up to the immigration officer in Thailand, this the conflicting reports. Rent is cheaper, but space is smaller as well. Cost per square foot/meter is actually about the same. Food prices are reasonable if you like to eat out. Western food at the market will be expensive compared to home.
Difficulty of opening an account depends on the type of visa. With a retirement visa took several different banks and multiple trips to accomplish. Just be persistent
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