I may be wrong, but I'm pretty sure you have to go to a Thai consulate (outside Thailand of course) to apply for the non-O. And, yes, you will need the affidavit in order to do so. Darren, you have so many questions. Get it done professionally! It may cost a bit extra, but you'll probably save a lot of time and money in the long run....

Some people will tell you that you can do it yourself. Yes, maybe, if you can speak Thai or have someone who can do so reliably. I'd rather pay a professional agent to help you prepare everything PROPERLY the first time (remember, you also need to report your address to immigration every 90 days, either in person, by mail a month before the due date or online 15 days before the due date... if the website is functioning, that is).

The income letter is valid 6 months, which means you need to apply for your non-O visa (or renew your retirement visa) within this time. The retirement visa is usually 12 months. Some consulates only accept applications for your "income letter" by mail (which is actually much more convenient of course)...