I hope that you continue in counseling and set some boundaries.
In order for you to stop feeling like a doormat? There has to be consequences for cheating. Think hard about what you need to be willing to try to return to right relationship.
Wife letting go of the cheating partner might be one of the conditions if she wants to repair things with you.
1) You may not even want polyamory. You certainly don't sound like you want it thrust upon you just so she can assuage her guilt for the cheating. Why are you left holding the bag for her poor behavior? That's not fair.
2) Even if you decide you are up for polyamory? You might NOT be up for it with her cheating partner in the new poly network. You have no reason to like or trust them.
Again, you both might consider reading
http://felislunae.org/relationships-love/coming-clean/
The genders are flipped in the article with "Phil" being the cheating husband and the wife being the betrayed spouse. But I think it might help some.
In order to salvage the marriage, BOTH of you have to be working on it. Not just you. If she's just dialing it in, and not really being present? Not really participating in the marriage repair? Painful as it is, you may have to acknowledge it and come to terms rather than throwing energy down a back hole.
Def keep working with your counselor for what is healthiest/best for YOU. Whether or not your wife is part of the picture any more.
Galagirl