the Thai bank account must be in your name only. You can not qualify if it is a joint account with a Thai person. A joint account is only accepted, and very seldom, if both parties are foreigners, in which case it needs to be double the bt800,000.
they know which country you are in, and specifically in your case not being in the US by simply checking with immigration. You were stamped out of the country when you left, and if there is no entry stamp after that then you are still out of the country. They know which country you are in simply by the immigration entry in the computer system, and it is shared data with other countries.
concerning the bt800,000 financial requirement for the retirement visa extension, or the monthly deposits of at least bt65,000 every month. In order to use the monthly deposits method for the very first year, immigration requires a notarized income statement from your embassy, and the US embassy no longer provides that document, so you can only use the bt800,000 method for the first year. For every year after that, and having 12 months of regular deposits from overseas, you can switch financial methods, but you can not use the monthly method the first year. The US no longer issues this notarized document because Thailand requirements that the embassy guarantee that the wage statements are true, and the embassy will not do this. They only certify that you are the one that signed the document, and as such immigration does not accept it. On a side note, in 2023 Bangkok was the most visited city in the world. About a third of those people visiting want to live here. To deal with this Thailand sets financial requirements to limit those numbers. The more advanced Thailand becomes technologically, and they are getting very advanced, especially computer wise, they crack down on those people using loopholes, and agents to scam the system to stay here long term when they can't, or don't want to meet the financial requirements. Just keep that in mind if you choose the agent route to meet the financials
you are confused. That visa is only 60 days, then you can extend it for another 30 days. I think your confusion is in the fact that you had 90 days in which to use it by entering Thailand. If you actually look at your entry stamp in your passport it will be for 60 days, not 90 days.
as one example those that are still under the old retirement visa from the Vietnam era, when bt20,000 was all that was required for the financial requirement as still grandfathered in at the bt20,000, as long as there haven't been any breaks in extending the visa. There are still a few in that category. There are others concerning the original elite visas that are grandfathered in.