Mediation is not the time for aggression. It is to see if there is middle ground on which you can agree. If mediation is not successful your next option is to get a custodial evaluator hired to perform the custody evaluation. You and your ex can agree or you get an order mandating the evaluation. You prepay the evaluation - they can be expensive. But the evaluator will consider both of your situations and and "dirt." If the dirt is old and no longer relevant it may not have a significant effect on the outcome.
