the oa can be purchased as a single entry or multi entry for the first year. If you border bounce to get a second years use then you will need reentry permits if you wish to leave and return
Do you mean has the visa exempt changed from 30 days to 90? No, it's been extended to 60 days for the 93 countries visa exempt is applicable to but it can be extended by another 30 days at immigration once in Thailand
the issue was it had to be from particular approved suppliers and those on offer were not the best coverage for the price. Has been a couple of years since I last looked closely though so may have improved
If over 50, get yourself an o-a visa in the states and then you can open an account with no problems when you get here and you don't have to put money in a thai bank. However, if you are way past 50 you may find the health insurance you have to buy not to your liking but worth weighing up the pros and cons before engaging an agent