I have never run into this situation before much less this late in the game.

Have a couple that is not sure if they wish to file MFJ or MFS.  One spouse wants to just file MFS to have the return done.

They want to know if they can file Extension MFJ then file the return MFS after the fact? or should the one spouse file MFS, pay their liability with the other spouse filing Extension MFS but if it is decided they will file MFJ can they?

Per 4868:  

If you and your spouse each filed a separate Form 4868 but later
file a joint return for 2022, enter the total paid with both Forms 4868
on the appropriate line of your joint return.
If you and your spouse jointly file Form 4868 but later file separate
returns for 2022, you can enter the total amount paid with Form
4868 on either of your separate returns. Or you and your spouse
can divide the payment in any agreed amounts.

I believe Extension MFS but later file MFJ and vice versa are valid options, but we are all over worked right now but maybe someone will read the fine print better than I if I am incorrect.