just remember 1) don't expect everything to make sense here and 2) a non trivial fraction of the people in these groups are sociopathic liars. Good luck!
My wife and I have had mixed luck with this SORT of thing. I suggest you try in Bangkok first and maybe save the trip. Failing that, use it as an excuse to go on holiday to Roi Et. We had, for part of our marriage process, province 1 send us to province 2, province 2 send us to province 3, province 3 insist they couldn't do it, then province 1 did it.
we didn't know and used to separate. So maybe I'll give it a try... What I don't want is to do something wrong and just count on them being too polite to tell me. Hmmm...
Hey, just did this. I don't think you can get a "common law" marriage visa. But you can get married and get the visa right after. They might try to harrass you about if the marriage is legit- and you will need to bring witnesses- but if you are legit I wouldn't worry about it.