I've been told by a legit agent (one that just helps with paperwork, no fake funds and a MUCH lower cost) that they don't check about 95% of the time. When they do check you have to prove the money is legit but it could be from anywhere. In your case, you will have ATM slips and/or your bank statements showing the ATM withdrawals. This is the type of thing I like using a legit agent for, though. Some officers might say you need things a specific way but a legit agent can keep them playing by the rules and not sending you chasing around paperwork.
My father lives with us here since before covid. He's also over 70. I inquired several places (before covid) and there was no option for a visa based on his relationship to any of us. One of his grandchildren is foreign in an international school but the school said there was no option for a visa from that. One grandchild is Thai and my wife is Thai but two visa companies said there was no option for a visa from that. In other words, he was the same as any other retiree. It appears that the helpful answers regarding that path are, unfortunately, your only options.
This is only needed for the amphur. In my experience, immigration will only ever care about the papers given to you by the amphur.
I believe the discretion immigration has allows them to ask for new and exciting supporting evidence each year at extention time to keep things interesting. I gave up this past go around and used an agent. In and out with barely a word.
The Embassy and Consulate do these all the time. It is something the Thai government wants but isn't a standard thing in the US, hence the reason they have the service listed on their US citizen services page for Thailand and have standard forms for it there. For now it is working but could eventually go the way of the income affidavit, which was similarly unverified back when they did those.
They don't confirm it. This is from their website, "An affidavit is a sworn statement of facts, made voluntarily, and confirmed by the oath or affirmation of the person making it. Please note the Embassy and Consulate assume no responsibility for the veracity of the representations that appear in the affidavit. Only the identity of the individual making the statement is validated."
The forms for both Bangkok and CM can be found here:
I don't know if such a document exists inside the US. I got the affidavit of being free to marry from the US consulate in Chiang Mai. It is just one of the standard forms you fill out yourself, swear it's true with your hand up, and pay $50 to have it notarized there. I'm not sure if the Thai government would accept any documents not from the embassy or consulate. I was also divorced and had to supply divorce papers as a separate thing, but was told that isn't enough by itself.
As I understand it, various fees can be charged, such as for inactivity, but they aren't very large fees. The account would only be closed, though, if the fees manage to drain the account.