You've got alot on your plate right now, and it may help you to break it down into smaller pieces to deal with individually, and one at a time.
What was done to your husband is among the most damaging things that can happen to a boy. It has affected him in ways that he probably does not yet realize. Your instincts to be supportive are right, as thats exactly what he needs from you. You can't fix him, but as you said, you can move forward together with the proper help.
The fact that your husband became a child abuser may seem understandable, but is actually quite rare. Most abused boys don't become abusers themselves. If he was a child then, he may be less culpable than an adult would be, but the damage to the others he abused is just as great reguardless. Consider placing your focus on encouraging your husband to make amends to those individuals that were hurt by him, rather than labeling him a monster. He has the opportunity to help bring healing to them if its needed.
Did he carry this abusive behavior into adulthood? Its a reasonable quesion to ask, and one that you have a right to expect total honesty from him. But don't assume that thats the case. Give him the time to sort it all out.
Apparently the person who abused your husband was a member of his extended family. Neither of you has any obligation to sacrifice yourselves for the sake of family unity. If it makes either of you uncomfortable to be in this person's presence, then just don't attend these family functions. You're not required to explain why, but you have the option of telling the truth.
If you choose disclosure, understand that it will cause conflict within the family. Some may call your husband a liar, take the abuser's side against him, or write him off as a wacko.
Breaking the family silence may also bring your husband a sense of empowerment, and healing. Outing the abuser also has some other benefits. Its likely that your husband was not the abuser's only victim. And that person may still be actively abusing children. Disclosing may encourage other victims to come forward, potentially stopping any further abuse by him.
You have many decisions to make, many things to learn and explore. But it doesn't all have to be done today. Take it one day at a time, and you'll find it easier to manage. Good luck.
"But now old friends are acting strange,
they shake their heads, they say I've changed.
Something's lost but something's gained in living every day
....it's life's illusions I recall, I really don't know life at all. "Joni Mitchell