Okay... Sarara's plan has... "merit".
I say "merit" in the most... light sense possible, like... I can see the idea... but that won't work in their favor in the slightest.
Unless, in the next chapter, she says this treasure somehow doesn't alter the flow of time in any way besides the one intended... killing Shirato, and by default erasing En-no-Ozuno from "history", will have dire consequences.
His presence in the world has shaped many of the events in their time, even Azuma-no-Miya's rise to the throne depended on his presence in a few ways, to only mention one thing that could go completely off the rails.
This is not a plan... this is a desperate gamble without a "real win".
Sure... if they don't mind the timeline correcting itself and drastically changing things, then it becomes a "valid idea".
But I sincerely doubt that causing such harm to the "history" would be a good thing in any way, I think dealing with the side effects of Kozumi's meddling would be easier than this "plan".
And one more thing... considering how divorced Kozumi seems to be from the correct flow of time... there is a chance this could backfire horribly.