"you just drive into Mexico and stay anywhere for anytime" Surely you can't be that naive. But because you probably are, just go ahead and do that. Thailand is doing fine without you
Doubt you will be able to just keep coming and going on 60 day entries as this shows a "pattern" that you are living here and not a tourist. Many stories recently of people being denied entry as immigration is cracking down. I don't know the details on the multi-entry tourist visa, but I'm sure someone here can fill you in.
They may ask for further proof of location, so best to stay in a city that is served by the Hanoi embassy. For what it's worth, we didn't have any problem with Ho Chi Minh, 5 days to process
I agree. $30 a day for food for 1 person is crazy! We budget $30 a day, but that's for 3 people, and most days we don't even come close to spending that much.