HI Joyless,
So sorry for your pain, doubt and hard times. It should not happen with the one who should be our "steady rock".
I had a horrific time with the rumor mill, which continues to spin around, even though I slowed the water down by dipping out of public range and almost going into hiding for a time. It got so I couldn't believe my own parents and everyone was throwing advice at me and I couldn't think or breathe. Anyway...
Some things from that time period of a few months ago stand out more than others, but one thing is, to trust most friends.
The only person in the whole world who had nothing to gain by giving me the truth is a friend I almost lost because of my grief and lashing out, but she knew where I was coming from emotionally and held onto me like a life raft after she told me.
Note: She is a fellow BS!
Family did not tell me and even my own mother knew, so just because one relationship may be "closer" than another, doesn't always mean truth will come from there. In the books and movies it should, shouldn't it?
I will say to believe your very own gut, for you may know your husband best.
STBXH here said things that he hoped would minimalize what he was doing, the whole condom thing and he even claimed that OW got checked for STD's, but he flatly refused. He tried to say that "she was negative, so I must be and we used a condom, so no one will have anything", at the same time he said, "oh, that never occurred to me" and so on.
What I found too, is that if something happened that is significant, chances are good that you will become aware of it in some form or other. Oftentimes, it's been me finding out by myself, by accident or by slip up of STBXH, like when he was around more and taking things away he would drop things or forget a piece of paper from the cabinet, etc...if your WH does things on the computer and internet, you can find those things out, too.
Evidence and action is far more believable than words, which can be altered. Often evidence cannot.
You sound a lot like I felt at one time and from where you are, what I had to do was find my "threshold", a/k/a, deal breakers.
I still don't know all of what STBXH did, but I finally got to a point in winter where I had to shut off any word of him and I am still there. He is not part of my future and simply causes so much pain I cannot function, so I find that I simply shut off communication about his personal or daily life-I also find him disgusting now, almost physically dirty, and still have no idea how many women he may have been with now...as my own fog lifts.
I hope your fog lifts soon as you gather information. I agree with the posts that say to keep the info. close at hand, for when I would casually ask STBXH something, he would yell at me and once called me a liar. I was/am not.
I wish you well and a flashlight with which to navigate through this tunnel.