AKTHUALLY there are non-insulting uses, e.g. to “retard” something or saying something is a “retardant” (like a fire retardant). Yes, those are the same “retard” because if I understand correctly, that’s where the word originated from.
I also want to point out the circular logic you have going on. It’s a slur because it’s considered one, so it’s only use is as a slur, as a result, it only gets used as a slur, so it’s considered a slur, so it only ever gets used as one.
To be clear, that’s not disregarding what you said, but I wanted to point out a flaw in that logic.