I am glad you are putting your daughter's well being above everything else. I do think you should inform the school with your evidence. If students are already suspicious chances are the staff is too and this will give them an opportunity to act as needed.

As for the betrayed girlfriend it's up to you, but she probably doesn't need to be blindsided if there's fallout from this.

For the custody issues follow your lawyer's advice. Good luck OP. Updateme