All down to the individual IO as you present. Its a risk that you need not take by getting the visa, and as its an e visa you don't even need to be incountry to collect it.
But even with a visa the individual IO can refuse entry.
Visa waiver entry then change to retirement can still work but success does depend obtaining a Thai bank account, which is now more difficult, and having all the correct required documentation to be processed timeously.
Not ideal, and yes the out of country retirement visa can be easier, but that does require health insurance which can be a problem for some.
The in country retirement visa does not need health insurance. 'That's what the b800k is for' quoted to me by an IO.
I'm unclear as to what the £12k mentioned at the end of your post refers.
If its the Thai entry requirement of b20k or equivalent that is separate to the capital or pension income requirement for a retirement visa.
If its the pension income requirement of b65k monthly that must be certified by the relevant embassy in Thailand (which the UK and US Embassies will not do, so that option is not available to UK or US passport holders).
Yes. If you don't intend to leave Thailand in that year, just b2000.
If you change your mind you MUST remember to buy a re entry before you leave for b1900 (can usually do it at departure immigration) but if you may go out twice or more the multiple entry is cheaper at b3800 (but I dont think you can do ME at departure immigration).
If you leave without a re entry the visa is cancelled.
Very strongly suggest you thoughly check out the region you intend to stay as regulations appear to have different interpretation regionally. And there is much variety in life here.
While information is available on the official Thai Embassy website for your country it may not be fully up to date (UK one is not iirc.)
Also be aware that some rules change rapidly and what worked a month ago may not now.
All out of country applications are now made through e-visa online.
Apply for the 90 day retirement entry
Iirc:If capital qualified, proof of funds (b800k or equivalent) must be in your sole name, not a joint account.
If b65k monthly pension qualified, you need certifcation of 'pension' source. ( I shall not cover this method as I am not familar with it.)
Health insurance must cover 1 year.
As soon as you get to Thailand your first priority should be to get a Thai bank account as this may be the most difficult aspect. (An 'introduction' from a respected local bank customer may help)
When relevant funds are transfered into your Thai bank account it needs to be shown as foreign exchange transaction (FET) or it may be questioned later.
Those funds then need to 'mature' untouched for at least 30 days before you can apply to have an extension of the 90 day visa for an additional 12 months (the earliest you can apply for the extension varies regionally so check).
You will then need to present further proof the bank funds have not gone below b800k at 3 months after the application for extension, and maybe sooner too.
Do NOT let the funds shown in the account drop below b800k for even 1 day in the first 3 months.
Month 4-8 the funds can drop to b400k but must go back to b800k at month 9 to apply for renewal.
If you arrive on a 60 day visa waiver or other 'tourist' visa you are likely to find it very difficult to open a Thai bank account which you will need to apply for a change of visa. Reportedly even agents are finding it difficult now. It does vary from bank to bank and region to region.
Without a Thai bank account you will not be able to apply to change to a retirement visa.
As I mentioned earlier I am not familiar with the b65k monthly pension procedures but do know that in country applications require certification through the embassy of your country and this is not available to UK or US passport holders.
All the above is based on my recent experiences but the rules and implementation do seem to change rapidily and regionally.
Almost, the b65,000 has to come from a recognised certified 'pension' source (but US & UK Embassies no longer give certification) and that b65,000 must be deposited each and every month of the visa's validity or the visa becomes invalid, and you have problems (like rejection) on renewal.