I don't know where you are reading but on the main GOV website (https://www.passport.service.gov.uk/overseas/information/thailand/renew/adult) nothing has changed. You still fill in the form and VFS then send it with the application to the UK.
The question with the Thai debit card is whether your bank will accept an overseas "cardholder not present" transaction. HMPO will not have any problem if that is the case.
It had to be paid by card. Payment is taken in the UK. It doesn't have to be your card. Or your daughter could send you your new card details although you will likely have to activate it somehow.
Well the embassy would have already issued that years ago. But in any event getting a new document proving you're not married is not going to help much with getting a marriage extension.
https://www.keyvisathailand.com/services/renewing-uk-passport-thailand/ is another possibility. Pattaya based but that is not an issue as you only have to send your documents to them by EMS.
If TG's system is in any way "clever" it won't allow the booking as the passport expires before the flight. Of course the only way to find out is to try it.
The OP's issue is that his current passport expires before the initial flight. And as TG ask for passport number and expiry date when booking, presumably their system would not be able to proceed to completion.
Indeed the case. I would always say getting the non-O in your home country is better, it eliminates the need for a trip to immigration for one thing, and the minor airline issue.
As is widely reported these days, opening an account without a non-O is nigh on impossible. However, getting a non-O in your home country is easy. Doing that you make opening a bank account easy and avoid airline issues with onward tickets.