As far as an affair, I know things can happen. If it changes anything about how I feel about the marriage - well, let's just say it would hurt me if she did, but I am still wanting this to work - call me a fool but I love her that much. For me to investigate it, knowing me, I am not sure it would help my case and my emotions may get the better of me.
That's fine so long as you operate more under the assumption that she IS, than she ISN'T. I don't see you doing that, but that's your business, not mine -- I just wanted to warn you to be careful.
There are things that differ, strategy- and tactics-wise, when infidelity is involved, and there are CERTAINLY some things -- legally, financially and even medically -- that are prudent when there's a third person in the picture.