My theory is that at least part of this will involve ensuring that Evie doesn’t get the System in the first place.
Previous chapter had future/alternate Elsa implying the goal was changing the future from Yvonne's perspective, not her past/history/present (with mutual confession and all.) I.e. Yvonne can't change the monster attack we already saw, but could prevent the ones 'Elsa' warned her would keep coming.
So in line with that, she wouldn't prevent the System from having happened, but she could theoretically do something to cause it to break after her departure/return time.
Of course 'Elsa' seems out to change her past, so I dunno. The author has put some thought into timey wimey metaphysics but we have yet to see the dismount.