I remember evading such a thing in the game before. I was a faction leader. Occasionally, a faction subordinate comes to you asking to be given orders. You can order them to assassinate someone. There's a chance that they fail or get caught, but that's how I evaded a similar event. Reloaded an earlier save and had the mastermind assassinated months in advance. Then I never got the kill-ruler event even years later.
If you explicitly want the fail event where the girls become prisoners, you aren't going to get a different outcome by reloading the same save. You'd need to reload a save from the previous turn and do something different from what you did the first time on that turn. That'll give you a different seed on the kill-the-ruler event turn. The different seed might give you a different event outcome.
Then just keep reloading to the turn before and do something different on that turn until the event fails. If the event never fails even then, that means their success variables for killing the ruler are just too high for them to fail.
