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.