The trope of the fl following the plot in Isekai, if she doesn't cave immediately, exists because it makes sense.
The concern is the Butterfly Effect. Where the smallest of changes can trigger a million bigger changes down the line.
Romance is often what leads to the mc being able to survive in these plots. Especially in this case, changing the target matters.
The fl already knows the ml can be happy with the designated love interest.
Also, if she does get with him and they end up happy and alive, there's no guarantee the fl would be able to do the same. Her destined partner is gone. Her destined future is gone. There's no guarantee she'd be able to pave a new way.
The stupid thing to do is actually to just go crazy and take the male/female lead of the original. I think they even showed how that goes in Cheating Men Must Die.
I'm more concerned about all the convenient falling down. Isekai fl's have serious balance issues. I said 'oh come ON' out loud this time when she tripped and the hood happened to fall off her head. I mean REALLY?
Just wanna let everyone know the RAWs say this series will go for 147 chapters and then there's a side-story.
RAWs are here: https://page.kakao.com/home?seriesId=50790508&
Official English Translation (same as here): https://www.tappytoon.com/comics/villainess
How are you going to title your isekai like this and write it like a thousand other generic isekai's out there?
More than that, this is a villainess isekai, which usually means the fl is able to stand on her own. This villainess is weaksauce shoujo trash. The ml is the domineering type that was played out back before women had the right to vote. The only thing they've got going between them is that she likes his body enough that SA doesn't bother her apparently. Replace him with a cardboard cutout he'd have more personality.