You have some advantages here.

1. Your W is young, and may cater to her parents influence, I can't imagine they would approve of an affair and a divorce, particularly if you two have a child.
2. Her parents likley won't approve of the OM if he's been this invasive. Try to get some dirt on him if you can.
3. If she's religious, which it sounds like she is, then that can influence her decision too.
4. Her being religious, you can turn to clergy members to counsel her. Most religions discourage divorce and encourage you to work on your marriage, particularly admonishing affairs.
5. She has'nt moved out yet.
6. You have a child, another influence to keep her from leaving.

There's a lot of positives here to be optimistic. DON'T listen to her...when she talks she is going to put up HUGE EMOTIONAL WALLS to fight you off and keep you out.

Maturity is the strongest force to break down those walls. Do'nt let the walls scare you off or intimidate you. She WANTS you to tear them down, but you can only tear them down by showing her you are an adult.

Keep up the good work.

Again look into that part time volunteer work. Find something she likes and volunteer part time in that area. Use your imagination. Showing maturity is what will win her back.