with a non-immigrant long-term Visa he gets a five year otherwise just a two year. I know because I have felt that pain going from five to get back to two.
The normal Cambodian border is a hotbed of issues. People not being let back in. Sometimes having to get down to Phnom Penh and flying back. I would suggest other options for exiting and re-entry.
14 years as a real volunteer. I know more than most immigration officers and I have regularly voiced my concerns about immigration dodgy practices with arrangements with some companies even connected to visa groups.
Actual immigration officers on the take just get moved.
Any amphur (amphoe) district office is all you need to visit and it's easy to arrange paperwork. Your Thai fiance should be able to get all the information.
Plenty of options but look for them specifically in province you want to live in. Don't get a visa from any other province no matter what any agency says.