Can I apply for a 12-month Non-O retirement visa extension before the 2-month bank deposit period ends?

Nope, you cannot 🙁

The seasoning for banked money method on retirement extensions is "the 800K baht must be seasoned in a thai bank account in your name only for 2 months on the date you apply for the extension. <- That's 2 months date to date.

That means until you actually have the funds in the account for the full 2 months you can't apply for the extension of stay