Since an attorney back then told you that she absolutely could do that, then most likely the overall conditions of the situation did allow her to do that.
However, even if she somehow did it when she really did need your consent, it is far too late to do anything about that now. You already admit that it would be unreasonable to change his name now (and the court would never allow you to do it anyway)...and it would be long past any kind of statute of limitations to do anything to mom, even if she had committed some kind of crime in doing it.

