What are the requirements for extending a non-immigrant O retirement visa in Thailand, including income verification?

Seeing as your embassy DOES issue the affidavit of income from abroad notary letter you can contact them to see how you get it and it needs to state you get at least 65K baht a month in income from abroad.

Also seeing as your embassy issues that letter you cannot use monthly transfers to meet the requirement you have to use that embassy letter.

Embassy of Ireland: Pension - income letter

IF your embassy issues an affidavit of income from abroad notary stating you got 65K baht a month in income you could use that OR if they don't you would needed to have transferred into the country 65K baht each month every month into a thai bank account in your name only for the previous 12 months

Only ONE person needs to meet the proof of funds, and the spouse would get an extension based on being married to someone on a long term extension of stay. Called "trailing spouse extension".

There are 70 immigration offices in the country and no "one size fits all" list of what you need

GO to your immigration office and get the hand out that lists the requirements for BOTH the yearly extension based on retirement and the one for the trailing spouse extension. That way you know if you can meet the requirements or not BEFORE you go to apply for your extension.

Extensions based on medical means can be issued for no longer than 90 days at a time, and you need a letter from a doctor or a hospital stating you are unable to travel for the period of time you're asking for your extension.