Anonymous participant 937 Nong Khai was the easiest border in Thailand until recently. If you want a corrupt border look at Poi Pet before they closed the borders.
What does the website of the embassy you applied through say? Every embassy has different processing times and even different visas have different processing times at embassies.
You don't apply for anything through immigration. Visas are handled through embassies, not immigration. You must travel to a country where you want to apply. Then you apply through that embassy on the e-visa website
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which is the ONLY way to apply for a visa now. That application will ask you where you physically are, and will ask for proof that you are there and may be further confirmed by the embassy.
As I said, each embassy sets their own requirements. Some want 3 months of statements, some want 6 months of statements. Some only want the last statement to show required funds, some want every statement to show it.
METV gives you a 60 day stamp every entry. Each entry can be extended by 30 days. But you stated you wanted more than 90 days, so whether you came in with a visa or not, you would need to leave and return at some point because both visa exempt and tourist visa give 60+30.
The source of the funds shouldn't matter for most embassies. Though he'll need to figure out how to get the extension in Thailand if that's the plan, because he won't be able to use monthly income there.
Get the non-O visa before you travel to Thailand. It will make opening a Thai bank account possible if you don't already have one, and will also save you 2 trips to immigration if you do already have a Thai bank account.
And if you didn't get married in Thailand and your marriage isn't already registered in Thailand, you need to do some things with your marriage certificate in the UK before you leave as well.
TM28 is a change of address form. It's mostly been retired, but some immigration offices still use it. If they're saying they require it, there's nothing you can do but meet their requirements.
If you're not in your home country then you cannot apply for the METV from there.
But that is irreverent because you would still only receive a 60 day entry stamp as it's a tourist visa. You're going to be have to do a border bounce to get a new stamp either way.