Maybe instead of deciding whether to believe them or not believe them you could try to communicate with the alter to get more of a dialogue going.
I know from my own therapy my T(s) have always said it is completely unproductive to focus on whether or not alters are telling the exact truth of what happened to them. It can never be known (or rarely, if there is external verification) so there is nothing at all to be gained from trying to judge it except misery.
What IS important is to develop communication between alters and self-compassion.