While you probably have a true case, is this something worth spending $10k or more of your money to end up with nothing? She is 16 and doesn’t have much in the way of money…not enough for court and not enough to even cover the expenses you’d have to put out to take her to court. You could potentially sue her parents, but that would be much harder to prove and even then you’re most likely talking about getting very little money in return. Add to that the depositions you’ll have to give and questions you’ll have to answer on the stand about your marriage, your sex life, and your personality traits, and it seems as though it’s hardly worth it. Her attorney will do everything to paint you in a bad light.
My advice is virtually the same as don5327. Change the locks, tell your husband that he can’t return, and get alimony and child support. As far as the repulsive emails and web postings (are you searching the web for her pages?)…I would just not open them, however, put them aside in a separate folder on your computer for storage just in case you need them for a divorce hearing. Proof of the affair will help with determining alimony (although it isn’t the only factor).
Move on. Just be thankful that he didn’t do this after you had been married for many years. Happiness is the best revenge, especially against someone who is so obsessed with competing with you as this young girl is. Instead of dwelling on yourself as the victim and your anger, build a full life with your children and show them by example how to rise above adversity, live an honorable life, and succeed.