What I recall is that Ymir was originally a pure Titan, then she accidentally ate Rheiner and Berthold’s friend, becoming one of the nine. I think the clock only started then.
The marriage comment was fine, but to me it felt a little like the writers trying to make the show topical. It took me slightly out of the moment just because it seemed they were trying too hard.
(Also it’s never been quite clear to me that Historia reciprocates those feelings the same way, but I admit I don’t catch every detail of characterization. )
Ymir is going to flip sides before the end, I am positive.