First let me say that I am not an expert in this subject, but I also do an extension based on Marriage, in fact I got mine last week (Khon Kaen) , we don't use an agent, we do it ourselves, it is easy, especially after you do your first one.
I have never provided a monthly statement.
In fact last year there was a confusion in my bank deposit. We had recently bought an investment property and some how, (wife was handling the finances) the account fell below the 400k two weeks before I was due to do the extension, so I told the IO . then lets do the monthly income method.
He said , do don't worry just come back in two weeks when the funds are seasoned and we will do an overstay. I express concerns about reproductions , he said not to worry the is the one who handles the issue and no problem,, so two weeks later I paid a small overstay fine and was good to go.
IMO the 2 month seasoning period for the 400k is so that someone who just came to thailand to be with his wife, would have enough time to season the funds before he need to do an extension.
Requiring a whole years deposit would eliminate that option, and no one would be able to do their first extension because no one would be able to open an account and stay here for a year to have a years worth of deposits.
There must be some confusion, either on your part ot the agents part.
I am sure they have a form but it could be the form nessacery to do the income option.
This year I did my extension a month before the due date. I think you can do it 45 days before, It gives you the time to resolve any issues before your due date
Good luck,
I am interested to find out how it worked out for you, so please follow up after your visit.