She is addicted to OM and if her C doesn't give her the tools, she will continue to be on this merry-go-round. Yes, I believe you are still her back-up plan. She is pregnant with OM's baby and is an emotional wreck. Most pregant women, IMO, would seek a safe place......and you represent safety to her.
Sorry, but I disagree with the other poster's C about people reconciling b/c that's what they want to choose, not b/c it's a backup (if I understood the post correctly). That's just not the case every time, especially with women...and especially pregnant women. For women, it is sometimes more about surviving, especially when they feel so vulnerable. OM has rejected her and the baby. Her passion may be for him, but she needs a sense of security. She knows you, Mr. Nice Guy, can give her the safety net she needs at the moment. So, yes you are Plan B. Her Plan A would not step up to the plate.
When I decided to stay in my M, it was not b/c that's what I wanted in my heart. It was not exactly my Plan A. It was more about doing the right thing and for the sake of my family. But I wasn't happy about it. So, could her feelings change after reconciliation? Sure, anything is possible. But it won't change without her determination to NC with OM. otherwise, I just don't believe it will be a true reconciliation of hearts.
If you decide to take her back, it should be under the terms of complete transparency. If she hesitates or resists........don't compromise. If she doesn't agree 100% to be transparent with her phone & computer (at the least) I don't think your M stands a chance against all that challenges it.
But let me ask, is she saying she wants to come back......and then tells you she hung out with OM after supposingly ending things with him? Or, did you discover some other way?
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!