You do need more docs for DTV than say the "O" on Retirement and Spouse but process seems to be similar for those doing it themselves. I am sure you will get extension when docs updated and you know next time. Good luck but I am sure you will not need it.
5 years in Thailand already on what must be assumed Tourist Visas and Exempt. Evidence all over the internet he has been working here including promoting real estate, relocation services and visa advice. The answer seems pretty obvious.
That is a good idea. I know a Thai clinical research professional just returned from China. I bet she is pi55ed off she has missed out on that. There are others I know in Belgium and UK. This is a good deal for them.
You can get a lease on any visa, including Visa Exempt. As for the length of lease, most landlords want at least 6 months but prefer a year. Any shorter and you are paying a premium, and many agents do not want to work with you. If you really want short lets you should be looking at Airbnb or serviced apartments.
There have been a couple of posts recently about extension being denied in Chiang Mai but I do not think they mentioned the fee if I remember correctly. The two of them were quite recent ie last few days