Depending on who you bank with you should be able to download 3 months as a single document. My bank allows for standard or certified save the file then in appropriate section upload the file to the application
a non O give you 90 days not 1 year if you have satisfied the requirements for the Non OA then yes it's valid for a year. There is a desk at the airport where you can purchase a re-entry permit. I would politely suggest you speak with them show them what visa you have and they will advise accordingly
for either extension applied for in country my belief is that your funds must have been in the account for 60 days I maybe wrong it might be 2 calander months
I am doing exactly the same I have applied for a straight Non O visa. My situation might be different as need a CNI aswell as all the other bits. Fortunately I am financially stable enough to convert to Non OA or Marriage visa depending on time frame availability of appointments at embassy etc. But the way forward is straight Non O get the bank account open get the money into Thailand then look closely at the requirements for each category you wish to apply under for the 12 month leave of stay. The critical part is both require the money to be seasoned.