As you have a non-OA visa, you have also the obligation to have a health insurance.
Yes, you will get a stamp of stay for 12 months (maximum) when your visa is still valid and maximum the end of your health insurance validity. Often the health insurance gives the maximum stay, as the new certificate you get normally 1 months before your new insurance is starting.
if you leave now and you re-enter before the end of the validity of your initial non O-A visa, you dont need a re-entry stamp (you will be stamped in for another 12 months or end of insurance validity).
If than after you leave Thaïland, you must buy a re-enter stamp to keep alive your visa.
you book another flight. The e-visa is valid for 90 days (is a multi-entry visa) and each time you enter within these 90 days, you will be stamped in for 60 days.
Without any abusive travel to Thaïland no immigration officer want this proof. Maybe they will put you away and than ask you to have the right visa to stay.
Yes.... for the airline company you might to show the ticket leaving Thaïland within 60 days.
Bangkok Bank you can ask via their application a 12 months statement for a saving account and send via email within minutes.
Unfortunately you can not do it for a fixed account. Therfore you need to ask it at your branch and it will be send by email the next 3 days (cost 100 bahts).