the website is generally generic, and asks you where you are physically located. Once you select where you are, it will tell you which embassy will process the application. Then you upload the information and proof of location
so that's what happens at your immigration office since you're probably in the district covered by Chiang Mai. But not every marriage extension in Thailand goes to Chiang Mai.
they're not all sent to Chiang Mai. They are sent to the district headquarters. I believe there's 5 districts. Maybe 6. They used to be handled at all a local level but there was too much fraud going on so they centralized processing and approval.
He can do it until he can't. Immigration will not allow him to live here without a long term visa. It's up their discretion on every entry whether they let him in or not.
It's still better to try each time. There are people that get approved and save a trip to immigration. If you're rejected the worst is you're out 5 minutes of your time on the computer. The official policy is that you must do it in person the first time after entering Thailand, each time you enter.
Lawyers say what will get your business. They could care less what happens in 30 years from now. Just look at all the lawyers who say setting up a nominee company and even providing the nominees for owning a house is legal. Until the investigators come calling.