never has been health insurance or police check for Non O, only OA. 90 day Non O is easy to obtain but have read reports of people being unable to open a bank account on this visa.
Probably ok but not guaranteed. Only guaranteed way is to get a Non OA retirement visa before traveling. This visa has mandatory health insurance requirements so maybe that's why they are making difficulties for the 90 day non O.
you can do this on METV with no hassles at the border. The border can be with a neighboring country or by air. It is possible to do the same utilizing 60 day visa exempt but can possibly be hassle on re-entry at the border.
have extended at HH for the last 8 yrs. You need to copy all the available bank book pages for the year but it is not a problem if you have not updated the book regularly just update on the day you go and get the certificate letter. The letter and update is good for 7 days.
in that case your visa has not been activated. You need to exit and re-enter the country to get the Non 'O' stamp in your Passport. Without the correct stamp the Bank will not open an account.
Be a bit wary about asking Immigration for advice, the visa is nothing to do with them and you are not supposed to apply from inside the country.
You need to check with your immigration office as policies vary but at Hua Hin office only the bank statement and a copy of the final book balance along with the certification letter matter.