This is the way I look at the decision about going to the C sessions. The "forgiveness" problem is your W's.....not yours. You know if you go to the session that you are going to have to hear her drag all the negative things about the R up all over again. Then you will either have to defend yourself....and if you do, it will make her mad......or sit there and validate. I really do not see the benefit in it for "you" b/c you are not the one having the problem. You more than likely will walk away feeling very low-down.
If I were you, I would tell her that you've gave a lot of thought to it and you feel that she should talk to the C privately instead of having you there with her. Just be kind but firm and don't back down. If she tries to argue about it.....DB her.
You are right.....you've been GAL and now you are just beginning to be able to see things a little better. Before, you were so down on yourself that it could not have possibly been healthy. Just as I don't see where you attending those sessions will be healthy for you. They hopefully will help your W, but you don't have to be there for her to learn how to forgive you. That's what the C is for.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!