Ok, say like in Spider Man 3, it turns it was someone else who killed an important character (in our case, Mama Longneck)?
I mean, there are ways it could be done.
1.) We don't know what happened before Mama Longneck confronted Sharptooth. Sharptooth might not have hunted alone.
2.) There was a sort of time gap between when they are running after Sharptooth's fall and when Littlefoot finds her. Perhaps she might have survived Sharptooth's attack, but her other attacker, who had somehow survived being knocked away by her earlier, and didn't want her to live as she had seen his face and knew his secret, had come and attacked her so that she would not survive her new injuries. Then, she tail whacked the guy, and he appeared to have fallen to his death, so she never bothered to tell Littlefoot about him. He then climbed back up a vine and was going to eat the fallen Mama Longneck and go after Littlefoot. However, other Leaf Eaters, perhaps looking for survivors of the Great Earthshake, were coming, forcing him to flee. The bad guy was closing in on Littlefoot, but had lost him when Littlefoot went to Rooter. The bad guy eventually found Rooter, but Rooter wouldn't tell him anything. So, the bad guy, determined not to let a victim get away, decided that he'd knock off Littlefoot if he ever found him.
Would that scenario ever work in LBT or would having that happen, so late in the saga, be a very bad idea to change what and whom killed her?
I know it's not impossible to have it happen in LBT as we suddenly had Bron come out of nowhere in film 10.