2 John does not say she was a bishop of a church,,,,,even if she was this letter was not from Paul and did not contain his doctrine. If it is your view that women church leaders were and are forbidden, then you will have a problem with John 2 commending the elect lady who was leading that church in her home, and who John was teaching how to recognize those whom she should not accept into the fellowship.
Even so their were churches under Paul who met in womens houses........[Act 18:2 KJV] 2 And found a certain Jew named Aquila,[G207] born in Pontus, lately come from Italy, with his wife Priscilla; (because that Claudius had commanded all Jews to depart from Rome:) and came unto them........Priscilla was a woman.........[1Co 16:19 KJV] 19 The churches of Asia salute you. Aquila and Priscilla salute you much in the Lord, with the church that is in their house.........the couple had a church meet in their house but doesnt mean she was the bishop of it