Exposure to a person really depends on the kind of person they are.
If you have an OW who is attacking your home... her parents RAISED her... I don't think you can expect much good to come from them. Good parenting raises people to respect other people's homes, not trod over them like dirt. The OW parents raised her with her values... it makes me doubtful they would put much pressure on.
HOWEVER.. this WOULD take the battle to the OW's court... put her on the defensive. If you go to HER work, HER family home, her FRIENDS and expose it will give her a taste of what she's doing.. That in some cases may help... it really depends on the people involved.
Your H is in a terribly awful position, but its a bed he made for himself. I honeslty wouldn't tolerate any visitation of H with OW without a chaperone who is marriage-friendly to you. That would be my first boundary. The SECOND is that the OW does NOT need to be at your H's visitation with his son... this CAN all be done via intermediaries... its not and that increases risk.
Your husband has trouble saying no and making decisions... That's obvious. I am going to say the same thing here as I think I already covered in mb28's thread.
STATE your position. Then LEAVE. Do NOT repeat the position... this guy WANTS to connect with you, even if its a fight he gets an emotional reaction from you and that's what he's looking for. Do NOT give him that. Give him silence.
If he pesters you ... LEAVE the HOME... call a friend and tell them what he's doing
example :
YOU : Hi Sharon. Yes he's lying again and it's stressing me out. He's supposed to be protecting our home he's my husband. This woman is trying to break us up and he's not stopping her. It hurts so damn bad. Can I come over there for a while? I want to feel safe and while this woman's attacking our home and family I don't feel safe here. Hopefully H will realize he's made a commitment and will send this woman away. Right now I just need to feel safe and I need help.
Ignore your H, he will interrupt the call or try... just get OUT of there... silence is all you offer.
You state your position once, maybe twice until you are sure he's heard it... after that..do NOT repeat it..he's just trying to get an emotional reaction... don't give him one. Emotional distance is the key...
if he wants that connection, he needs to man-up. And he will pursue you to get it... just hold onto it and don't let him have it. MEN are much better at distance and the silent treatment usually, so you have a disadvantage here... don't let him exploit that... state your case and then SHUT OFF the channel... WALK AWAY.