Huh? The Old Ones would logically want her gone, so she has to be more vulnerable to make it happen more easily? I dont follow your logic.
If the Archive's existence acts as a deterrent to the Old Ones returning, and that existence can be terminated, they would attempt to terminate it. This is the case where the Archive ceases to exist when its bearer dies without a specified heir.
If the Archive's existence acts as a deterrent to the Old Ones returning, but that existence can only be periodically suspended, they would only target it during times critical to their operations. This is the case where the Archive finds a new, indeterminate bearer when its current bearer dies without heir.
I was thinking along the lines of effort put in vs results from their point of view. If they can get rid of the Archive permanently, then the Old Ones would be salivating for a chance at Ivy. If they can only temporarily put the Archive out of commission by killing the host, they wouldn't risk as many resources/personnel unless the timing of the Archive's absence was especially opportune.
Does that make sense? It does in my head, anyway, but considering this post deals with the concept of Mantles yet again, I'm already mentally fatigued. Could be mixing things up.