1- From UK so you can apply for the non-o at an embassy (Cambodia, Laos, Vietnam, etc..) for a 3 months visa. You will extend it to 1 year in thailand.
Problem is that each embassy got different requirements
2- It’s 90 days... note that you will have to do it separately...
The same kid can’t have 2 guardians.
3- As I said in question 1 each embassy got different requirements. In Cambodia I had to show my bank statements. Can’t remember the exact amount they asked... But I think it was 10k $US... after that (because I did it several times) , sometimes I had only 5k and it was ok.
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5- I think the bank account name will be a problem.
As I said, 1 kid can have only 1 guardian so it will be treated as 2 separate requests ... so you will need the money seasoned in a bank account at YOUR name for the extension.
I did several 3 months back to back a couple of years ago... because I couldn’t meet the financial requirements for the 1 year extension (I can’t let sleep that much money).
But now it became harder... immigration officers don’t like it anymore.
6- and 7- That’s impossible to predict... it depends on the officer you see at the immigration.
But my advice would be to always have a B plan ...
I have been denied entry a couple of years ago at the airport and my kids were with the nanny in Bangkok...
It was like hell for me.
You can be sure that 2 parents en non-o with 2 kids in international schools can of course bring questions.
So my only advice is that you must be ready all the time.. just in case...
For example I prefer land borders... at least you can just go back , relax, and find a solution.
By sur you might end up in detention cell.