X2A lot of people say a lot of things online but their experience is generally limited to what is imposed by birds in confinement being fed diets the keepers do not understand. I keep my birds under a range of conditions which helps see how different nutrition and husbandry regimes impact behaviors and health.
I doubt the damage caused by other birds was traumatic enough to bust an egg internally. Change in feces could be because of change in diet or internal infection.
I am aware your are concerned about the bird currently in trouble which is a reactive and neccessary measure. Unfortunately I suspect you will realize this type of event again because you are not taking proactive measures that prevent such problems from arising in the first place. My birds get injured relatively frequently but it does not result in cannibalism.
Saying "it worked til now" is like saying you shouldn't put your kid in a car seat because your mother never put you in one and you're just fine.
Cannibalism is not a natural trait in domesticated chickens.