Can I enter Thailand on a tourist visa and then apply for a Non-O retirement visa using cash for the 800,000 baht requirement?

Okay, I deleted the bad info and the off topic comments

IF you are going to come here and apply for a 90 day Non-O visa based on retirement using banked money method the 800K baht has to be transfered IN to your thai bank account in your name only from abroad. Period, end of story, there is no wiggle room on that and it specifically states that on the hand out they give you for the 90 day non-O requirements based on retirement.

IF you are going to buy the 90 day Non-O visa in YOUR country from a thai consulate before you wing your way here you show proof of funds in your bank in your country.

Either way, once you have the 90 day Non-O visa you need to have the funds season in your thai bank account in your name only for 2 months before you apply for your yearly extension based on retirement AND at that stage (getting the year extension) there is no proof of origin of funds required.

You can, if you buy a non o before coming here. If you come on tourist visa, or visa exempt, and want to buy the initial 90 day non o inside Thailand, you will have to prove an 800k international transfer to an account in your name only. Customs declaration and anything else wont work. Only an international transfer showing up in your bankbook/bank account. If you already have the non o when entering Thailand, you can just deposit the funds at a bank, in an account in your name only, and have them there for at least 2 months for seasoning before applying for the year extension.

You will be hit by most countries EXPORT of cash. In principle tho yes ...provided converted in thailand with accompanying documentation. I did many years ago so situation may have changed.

I'd say no, as you'd have no way to prove it came from overseas