While you can purchase an OA visa while you are in your home country you can also return as a tourist and then apply for a non-O visa inside Thailand. Both eventually lead to a 1 year extension.
If she decides to purchase an OA visa in the US then those rules apply. She should be able to use money in the US and she will be stamped in for one year or the to the end of her current health insurance policy. She also, I think, will have to leave without a re-entry permit to void her current permission of stay. After that everything should be the same as before. She would not need to meet the 800k in a Thai bank requirement anymore.
As a strategy, returning to the US to purchase a new OA each year can make sense if you want/need to return to the US anyway and you are able and willing to go through the OA application process each time.