Will I get a marriage visa extension if my passport expires before the new extension period ends?

Okay let's forget the "I'm eligible to apply" part and instead just go with the date your current extension expires (because that's when your new extension will start from)

IF you apply for a yearly extension but your passport isn't valid the full year you get an extension only until your passport expires.

And as people pointed out that "under consideration" stamp is actually the first month of the year extension you just applied for. 🙂

The under consideration period is part of your extension so you should get the full year out of it

It would be no problem. But, You will be stamped until the earlier date: after one year later (new 1-year extension) or your passport validity.