Ths flight from BKK to Samui that the OP is on is a domestic flight with Bangkok Airways arriving at the domestic terminal in Samui where all passengers get off and walk through baggage claim to freedom without a passport control or immigration official in sight in this domestic arrivals area. Let’s not add confusion for the OP by talking about other airlines and other scenarios.
Too many guesses here. If you bought a through ticket on a single reservation then you will do Thai immigration at BKK and proceed to the domestic part of the terminal for your onward flight to USM. Your checked bags will go straight through to USM so you don’t need to collect and recheck at BKK however you will collect them from a different carousel to the domestic checked bags at USM. This is announced when you arrive at USM on your connecting domestic flight so I know it’s how they do it.
Not as bad as Dubai. They put a large stamp to enter right in the middle of a new page and a large stamp to exit right in the middle of another new page about five hours later when I was on a short layover. Deliberate. Malicious. Knowing. And I have a 48-page passport and travel frequently around Asia where I live so need every page. I’ve found Thailand’s stamps to be quite reasonable. Apart from one period when I did exactly what you did and tried to reserve some blank pages with post-it notes. I got turned away at BKK and had to relent.
You can get married. Easily and cheaply and legally with international recognition wherever gay marriage is recognised. We got married online in Utah last year. But it doesn’t help you in Thailand because they don’t recognise gay marriage. So get married if you want to. Very straightforward.
I’m guessing that it’s because you have to upload your last 12 months passport history to apply for the AO visa in your home country and thereby prove that you’re applying for it from inside your home country. That’s one possible explanation.
Revolut also charges 1% exchange fee into Thai Baht because it's says it's a "rare" currency. Even for Metal cardholders. I use my Revolut Metal card to extract my limit of £800 a month in Thai Baht costing 2 x Baht220 ATM fees and there's always Revolut's additional 1% rare currency exchange fee. It makes it a bit more expensive than Wise.
Le Meridien Bangkok is pet friendly and will let you keep it for your T&G night. We brought our cat from HK to BKK a month ago. Shout if you want any help with the Import Permit (easy) or other details.