If the child-rescuer and child-murderer seem to be feeling the same emotions, having the same experience of righteousness, when imagining their opposite acts, would you still conclude that it is a mistranslation/misuse to identify our word "morality" with whatever word the righteous-feeling child-murderer is using for what appears to be the same feeling?

Depends. If the child-murderer knew everything about the true state of affairs and everything about the workings of their own inner mind, would they still disagree with the child-rescuer? If so, then it's pretty futile to pretend that they're talking about the same subject matter when they talk about that-which-makes-me-experience-a-feeling-of-being-justified. It would be like if one species of aliens saw green when contemplating real numbers and another species of aliens saw green when contemplating ordinals; attempts to discuss that-which-makes-me-see-green as if it were the same mathematical subject matter are doomed to chaos. By the way, it looks to me like a strong possibility is that reasonable methods of extrapolating volitions will give you a spread of extrapolated-child-murderers some of which are perfectly selfish hedonists, some of which are child-rescuers, and some of which are Babyeaters.

And yes, this was the approximate point of the Babyeater thought experiment.