Can I switch from a retirement visa to a marriage visa based on monthly income if my proof of income is delayed?

you just plain and simple don't qualify to get a year extension based on marriage using monthly income because of that mix up where 4 months were paid at once into the account. That voided your ability to do it

.

In order to change the reason for your extension from retirement/banked money to marriage/monthly income you would needed to have kept the 800K baht in the thai bank account in your name only for 3 months after your retirement extension was given, then the balance couldn't go below 400K baht the rest of the year

AND

you'd need to have deposited a minimum of 40K baht each month every month for the preceding 12 months before you apply for your extension. Switching OFF banked money method to monthly income is the toughest switch

Also you can't get the initial 90 day Non-O or the first year extension based on monthly income unless you legally work inside thailand OR you come from a country whose embassy still issues the affidavit of income from abroad notary (the US does not)

This means if you came back on a visa exempt or tourist visa entry you'd have to use the 400K baht banked money method to get the initial 90 day Non-O and then the year extension. It'd only be the following year that you would be able to use monthly income with 12 months of transfers.