If US citizens arrive without a visa they get a 60 day visa exempt entry like many other countries.
The OA visa is one of several long stay visas. You can only get an OA by applying from your home country. It is a 1 year multiple entry visa. It has a variety of requirements including the need for health insurance coverage.
You report to Immigration if you are in Thailand a continuous 90 days. If you are outside Thailand you don’t report. The 90 day clock starts again when you return.
Exactly. If you buy when you are young and stay with the company then they will cover you until you are 99 years old. This couple is unlikely to be able to buy adequate income at their age. They should consider whether they can self insure.