I am under the impression that Mikasa’s main role in the story was to show Founder Ymir how to get over a dude. So if she killed Eren but never moved on, that’s basically like pump faking Founder Ymir into making her think you can move on.
If Mikasa did not get married, in my opinion her entire arc doesn’t make sense with the story overall. She needs to move on, to be consistent with the Ymir’s whole “find connections and not be a slave to a single person thing”. If she did not move on from Eren, Ymir’s ghost is somewhere out there hella pissed that she got tricked.
And with the red scarf thing, hopefully she didn’t wear it for the rest of her life cause that’d be pretty weird (like imagine mystery man having to take it off her whenever they…poor guy) but I’m cool with her husband/family putting it on her when she’s laid to rest.