Like it or not, the Courts rule and the Court says that in this instance you cannot prove (by Court definition, NOT common sense) your case.
Your better argument (one that is being sounded across the country - no success, but cracks are beginning to show - do a search on Galluzzo - first custodial case to be accepted by Federal Courts. He lost, but the path is now established) is a Constitutional one, ie. States have no right determining custody when two fit parents who are willing and able to care for the child(ren) divorce.
Good Luck,
Still Learning