I think you had a positive outcome by how you handled things (at least it appears that way for now). Over time, things will show themselves. If what your mom is saying is true (and I truly hope that it is), then this could be a wonderful thing. Her response was not angry, defensive etc. and to me, that's a very good response. I know you feel like you lost trust in your mom. But if what she's saying is the truth, please give her the chance to at least try to earn back your trust. As for sharing with your dad at this point, that would destroy any possibility for your mom to earn back your trust and likely could be very damaging. Believe it or not, if things turn out poorly in the end, your dad might actually associate you (because you were the one to share the news) with the situation and he may be really angry and hurt by that.
I would say that you confronted this in a calm, respectful manner and received a positive response. Now keep on that path and let your mom try to earn back your trust and let things play out. Try not to hyper examine her life and try to let go of your feelings of violation on behalf of your dad. Both you and he WILL find out, over time, whether your mom is telling the truth or not.
Try not to carry this burden for your parents right now (so much easier said than done, I know) and give it some time. You seem like you were able to filter through everyone's advice to make a plan that seemed to work. Keep thinking like that and don't act impulsively before you make any further decisions about this and please keep us posted.