Can immigration request that the 400,000 baht be kept in my bank account longer for my Non-O visa application?

If you are seeing the list from Immigration HQ, it's for an initial in-country Non-O visa application for 90 days stay stamp. Right, you don't have to keep your funds once your visa is inked into your passport for that visa unless your immigration office asks. Of course, you'd better check with them when you apply. (Actually, you don't have to sit your money in your bank for two months for an initial visa)

BUT, if you want to apply for the following 1-year extension, that application does require you to have the same funds in your Thai bank account for more than two months prior to your application day as well.

This is general information for a 1-year extension based on being married to a Thai.