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.
The non-O visa is only a 90 day visa. It seemed logical to me, if I was retiring to Thailand, to get a 1 year multiple entry visa. It was easy since I didn’t need money in a Thai bank account. The documentation was substantial but straightforward. The process to enter Thailand as a tourist, get a non-O, and then get a 1 year extension seemed much more complicated. Why not sell an OA visa along with all the other visas sold from Thai consulates around the world?
At that time there was no health insurance requirement and there was no advantage to a non-visa for a retiree.
I had a qualified Thai insurance policy before they made it a visa requirement for the OA for that reason. I believe it is a good thing to have health insurance. I am just not sold on the concept of tying it to my visa extensions.
I came on an OA in 2017. The process to get it was fairly easy and it has some nice features during the first two years but with the recent health insurance requirement it has lost its luster for many people.