you need to GO TO THE IMMIGRATION OFFICE and ask them for their list of requirements. The forms you need are there. There's no point in trying to start early because immigration offices are known to look at any form you printed and brought with you and throw it away and hand you their form because you printed yours on the wrong size paper, or you printed on only one side and they want 2-sided, or you printed 2-sided and they want only one side, or you used black ink instead of blue, or they have a special form that other offices don't use.
Stop trying to use logic. Just go to the immigration office find out their process directly from them. Thai processes aren't about efficiency or what makes sense. It's about checking every box on their list.
There's no need to apply for an extension. You can apply for the retirement non-O visa directly.
You need to go to the immigration office and ask them for their handout of requirements to convert to retirement visa. Each immigration office sets their own requirements and they even change from time to time. So you'll want to get the list directly from them. Also ask them for the list of requirements for the 1-year extension since you'll be applying for that about 2 months later.
Then every year before your extension go back and ask for the handout for the extension again so you have time to prepare all your documents and make any updates if there were changes.
There's not really any need if you're in the UK. The embassy there issues visas in 2-3 working days.
You also don't say which visa you want to apply for. If you have a British passport you'll get 60 days without a visa, and can apply for a 30 day extension.
I'm assuming you're on a tourist visa or some other type of visa that isn't a long term visa. Almost no banks will open an account for you if that's the case.
It's not a fee. It's to purchase an insurance policy. No, you don't have to pay it but then they won't open the bank account for you. It takes then a large amount of paperwork to open a bank account for a foreigner and therefore they don't want to do it unless there is something in it for them.
Most other banks won't let you open an account at all, so the insurance is a workaround to get the account.
If you don't want the insurance you can pay an agent you help you open an account, but you'll probably pay nearly the same amount.
It's through whatever system the Thai embassy uses in that country. Some are e-visa and some are in person. Vietnam just switched to e-visa a few weeks ago.