He cannot use the pension for anything, because the embassy will not certify it. But during the entire first year he can transfer at least 40,000 baht (65,000 baht for retirement extension) to his Thai bank account from overseas. Then for next extension if he can show 12 monthly transfers, the immigration office may accept that.
He would need to check with them before starting down this process, as some immigration offices will not accept international transfers for marriage extensions. The only monthly income some accept is embassy certified income, or proof of income from a work permitted job inside of Thailand.
You can buy it either. But you need to understand what a re-entry permit is.
A re-entry permit keeps your existing permission of stay stamp alive if you leave Thailand. If you buy it before your extension, then it's only valid for the stamp you have at the time you bought it. If you buy it after your extension, then it's valid for that extension. All it does is make your existing stamp not be cancelled. It doesn't do anything special, there's no trick with it. It's very simple.
Immigration does not require proof of onward travel to enter Thailand. But you are applying for a 90-day visa, so you must give them information for a flight leaving Thailand within 90 days. It doesn't ask you for a TICKET leaving Thailand, it just asks for a flight. Go find one and put it in.
You do what you are supposed to do. You get the visa from the Thai embassy in London before you go to Thailand. That's how visas work.
Then when you arrive in Thailand you will have the non-O visa and can open a bank account.
The problem you are going to have is that you cannot use your monthly income for your extensions. The first 1-year extension based on marriage can only be obtained using 2 financial proofs.
1) Embassy certified income. The UK embassy has not offered this for many years.
2) Proof of 400,000 baht in your Thai bank account that has been there for 2 full months at the time of your extension application.
Because 1 is not available to you, only #2 is available to you. There are no other options for you.
You need to go to KASIKORN bank with your proof of transfers from Wise, and have them issue you a credit advice showing that money came from overseas. Include this with your statement from SCB and that should be all it takes.
You might need to find a Kasikorn Bank branch with a Kasikorn Bank International Trade Centre.
You need a Thai driving school, or an agent that specifically helps with drivers licenses. If you find someone who says they will get you a drivers license without going into the DLT, then it's 100% a scam, always, every time.
If you find an agent that will take you to the DLT and basically help you skip the line, then it's legit. If your license is issued to you directly by the DLT it is not dodgy.
There is no "reset." He will still have the same record he had. Immigration looks for patterns of people trying to live in Thailand without a long-term visa.