So you'll be getting your initial Non-O from outside Thailand then get your bank accound and funds seasoned for your first extension (assuming your Embassy does not do the income affidavit)
You say "this time" so that suggests to me that you are switching methods and applying for a new Extension of Stay based on marriage not a Non-O Visa. Is that correct?
yes but during the first extension you must keep 800k for three months then not less than 400k for the rest of the year as they were part of the conditions that apply to the first extension so, effectively, you have to meet two sets of requirements to switch
That is the rule for getting the 12 month extension, 800k for 3 months after then not below 400k for the rest of the year. They can ask for it and sometimes do as you have found out probably now to flush out the extensions granted with the aid of agent's funds .