He was not 'cheating' on you in the traditional sense of the word, with another woman, or several other women. I do think that was a good question to ask him though, but, it does not fit the character profile, generally speaking of a predator, who exploits children for sexual gratification.
You are dealing with a man, who has some serious mental health problems in my opinion. That area of his personality has nothing to do with your marriage. He needs to acknowledge what he has done, accept his behaviour and be truthful, and then seek help in changing his behaviour. Recividism is very high among adults who prey on minors. The three years in jail did not 'cure' him of his behaviour of your minor niece.
I consider the 18 year old to be a victim also considering her circumstances, and his role of 'responsible adult' in her care under your roof.
The residual effects to both these victims, and their families, will be a lifelong struggle. The punishment for the minor may pay, by having had a jail sentence, as far as the law goes, but anybody working with, or having family members, or are teachers of children who have been violated in this way, know that the struggle didn't end when the jail sentence did. But, it did for your husband, who went on to victimize a very troubled 18 year old. There were many things he could have done- research for one, or seek help with any women's service that assists those that have such troubled lives as that young woman did. They would have jumped through firehoops to advise and offer guidance and support. Hand's on support, with counselling, education, etc.
It would have been easier to accept, and try marriage counselling had he had affairs with willing adult females, who mutually and intellectually and physically met the needs of each other. The situation you face, is not that.
It is a part of his personality to work his motives on you, as he did with the two we've talked about. He is 'sincere', he 'says all the right things' he 'promises to change' he 'wants to save his marriage' etc. etc. Showing all the right manipulative signs, such as frequent texts, loving gestures and words, and 'remorse' are all a part of the same package his victims succumed to. That is the man he is, and you too, are being victimized by his behaviour.
He cannot change because he says he wants to, and you can't, or shouldn't expect that he can change either. That part of his life will take a lot more than a few visits to a counsellor. Marriage counselling will certainly not help him get 'the cure'. He is a sick individual.
I may be too blunt, and I don't mean to sound harsh, but it is important that you separate 'affairs' and what that implies (two consenting adults), to predatory behaviour against a minor, and against a vunerable teenager.
Clearly you are on the right path, but, I am not so sure that any questions you ask or demand of him, will have any insightful or meaningful responses. He just wants things to get back to normal. And a few crumbs thrown your way might just smooth things over enough so he can feel vindicated. If YOU forgive him, then he faces no other serious consequences- just lip service, a sad face, and a few superficial gestures, and his life is okay again.
No amount of explaining on his part, will change who he is.