Formally you’ll need an onward ticket within 60 days for visa exemption entries and especially your airline might ask for it. It could by a cheap ticket to a nearby destination.
If you don’t manage to get a Non O before you come, you might need to use one of the bigger companies in Bangkok -as the owners of this site- to assist you with the Non O visa process including opening a bank account.
There’s no official limit or regulations regarding this and everything is in the discretion of the IO you’re standing in front of. If they mean that you aren’t a genuine tourist but use tourist solutions for long term stay in Thailand you risk to get pulled aside and told to get a proper visa according to your travel pattern and/or denied entry.
If you only stayed for twelve days you should be able to re-enter as a genuine tourist and get another 60 days visa exemption and do a 30 days extension after. No subtraction is (or can be) done.
Yes, if he gets a affidavit letter as I understand you’ve pointed out. Some immigrations also ask to see the original dokuments used as documentation for for the affidavit letter.
But OP ask about the first year extension and income method is possible for the second year as you describe.😉 (And the formal requirements are > 65K baht.)