Hmm this is very true, though given Edward II's weak leadership, what's there to say that he doesn't end up getting replaced in the civil war either way?
Edward II was not a strong ruler, but legitimacy was not a easily lifted thing in Medieval Europe. As long as the Mortimers had no alternate candidate to the throne, they were in a precarious situation, as shown by the failiure of the insurrection in the Despencer War of 1321.
An Edward III being raised from 12 to 18 in such a troubled period would be a rather different person than OTL. I remember somebody her on AH has wrote a story of Edward III coming of age at the side of his father rather than his mother's. Need to check