No I don't think you are being oversensitive and I don't like anyone calling my DS 'naughty' and I agree I prefer to be positive about his behaviour. I found when they were spitting food or throwing it it meant he had enough, it does stop and they go through these phases like you said.

As for how to approach it with your mum that is tough. I would start by writing it down so you are clear in your head the sorts of things she can say so you have given her an alternative to the negative, explain that you don't want you LO spoken to like that?! goodluck