not even in this case would a tourist visa whitewash the fact that you used "touristic entries" for a longstay in Thailand. It doesn't make any difference - both the visa-exemptions and tourist visas get scrutinized if used "too many times" in a row
the exchange rate of today doesn't match these numbers, so I would only touch it with a pair of golden tweezers. He can however do the math himself on how many GBP will equal the minimum of 800,000 THb in today's exchange rate
the 90-days Non-Imm-O retirement visa is valid for use for a single entry within three months counted from the date of issue. This will be noted on the .pdf visa document under "visa must be used by" and the date
when entering Thailand on a Non-Imm-O Retirement visa, you will get stamped in for a 90-days stay permit. The visa itself will become void and invalid for further use. You should immediately get a Thai bank account opened and transfer the required 800,000+ THB. You can then apply for a 1-year Extension of the Stay Permit based on being over50/retired on Immigration as soon as your minimum of 800,000 THB have been sitting in your Thai bank account for 2 months - and you bring to Immigration the "bank letter of guarantee" which certifies this on the day you apply
when you apply for the Non-O Retirement Visa in the UK, you EITHER need the equivalent of 800,000 THB in your UK bank account, OR show proof of a monthly income of a minimum of the equivalent of 65,000 THB using original documents. . . . . . . . .however for the application to the 1-year Extension of the Stay Permit based on Retirement, your need 800,000 Thai Baht in a Thai bank account in your sole name since 2 months on the day of application, because you cannot use your income proof on the Thai Immigration (your embassy doesn't issue certified income affidavits)
well, and you are already on the "1-year Extension of Stay based on Retirement", then this 90-days Non-Immigrant visa from Chiang Mai in your passport is already invalid. I do not think that Immigration in Pattaya is going to transfer the 90-days visa stamp to your new passport, they normally only transfer valid stamps, which would be the extension stamp
Immigration will only transfer their own stamps. This means, they will transfer the existing "extension of stay" stamp and any re-entry permit you might have bought for it. . . . . . If you are about to re-enter Thailand, the "admitted until" stamp will go into the new passport, but you will have to show both passports and the "extension stamp" you are on, in order to get stamped in correctly until the expiry date of the existing extension. The visa itself was on a piece of paper, the visa you used before is already invalid. You are in Thailand on an "extended stay permit", not on a visa
the Royal Thai Embassy Vientiane has gained a small reputation as being one of the most easy-going embassy in the South-East Asian region. So their workload is predictably large. It is as it is. Vientiane is a nice place and you even could make a tour to Luang Prabang or Vang Vieng in the meantime while you wait. Even in the event you are invited for a personal interview, you can get back to Vientiane within hours by the new fast train between these locations