, you only keep 400K until the end of current extension based on retirement to keep it valid. 800K for two months is for meeting requirement to apply for a new extension based on retirement, if you don't apply for it, ot you apply for it for different reason, you don't have to meet that.
No, you don't need to have 800K baht for a 1-year extension based on marriage to a Thai. Be sure you can prepare the necessary documents for a marriage extension, if not you may have to apply for a 1-year extension based on retirement again.
Not sure what you mean by "12 months funds method". If you have funds of 400k baht in your Thai bank account, you should be fine.
If you mean 12 months of bank transfers, you need to check with the Burriram office whether you can use that for the first-year extension. If they don't accept, you may need to put the funds of 400K into your bank account and let it sit for 2 months before you can apply for an extension. In that case, you can apply for 60-day visit Thai family extension first, then apply for a 1-year extension based on being married to a Thai.
You are applying for a 1-year extension based on being married to a Thai, so no "minimum remaining days" requirement (that is for the initial in-country visa application, not for extensions).
So application submission tomorrow wouldn't be an issue.
You will need a Bank letter and passbook updated on the day of application.
If you earn monthly enough for a 1-year extension based on marriage (40,000 baht per month), you don't have to change the extension, you just use the monthly income method (by working legally in Thailand) to meet the financial requirement for 1-year extension based on being married to a Thai. In this case, you still need bunch of documents from your school.
If monthly income is insufficient, you have to change to a Non-B-for- teaching-based (not Non-O) visa/extension. You may need to follow the directions of your local immigration office and school.
Go to the local immigration office to ask if you can apply for 1-year extension based on teaching for next, and see what they say.
, first of all, you have to talk with your employer about how they support your wife’s visa/extension. You (your wife) need a lot of documents from your employer.
Check with your local immigration office as that policy varies depending on the office. Most offices accept a 1-year extension application when your current stamp is 30 days or less. Some offices including Bangkok, Chiang Mai, and Phuket, let you apply for an extension when you have 45 days or less.
You can do either. You just provide the air tickets all the way UK - Malaysia - Thailand for application in UK on eVisa.
As UK passport holders will be stamped for 60 days upon entry now, you may not even need to apply for a single-entry tourist visa unless you have a specific reason for getting SETV.
Don't forget to apply for a re-entry permit before you leave Thailand.
Show both old and new passports at the entry to Thailand. Make sure you are stamped in for the same date with your current extension with Non-RE on visa class on your entry stamp, before leaving passport control.
Once you are back at your address in Thailand, bring both passports to the immigration office that issued your extension to apply for transfer stamps.