I mean, I think given what OPs shown of himself here, your options are probably best, but there’s a 3rd that wouldn’t be so bad either:

Contact Christi privately and ask her what happened.

If she says yes, it’s his, or her timeline still leads him to believe it’s his and she denies it, he can still petition the courts for a test. They can test the child without her knowing what the swab is for. And then OP and Christi and the kids adoptive father can explain with a therapist to the child that she has two fathers who both care about her. Slowly do some supervised visitation.OP could become a presence in her life in a healthy way that doesn’t have to traumatize her or ruin her relationship with her ACTUAL parents without destroying lives.

But the fact that OP wants to run into this situation with guns blazing and snatch this kid away from a potentially happy and stable life leads me to believe he’s too selfish to be looking out for kiddos best interest and go this route.