Lass, You do what you need to do. Our love and prayers will be with you.
When you go to the MC, I would open up the dialogue with this: "I have recently discovered that H is a sex addict. Do you have experience with dealing with this and, if not, can you direct me to someone who does?" IOW, I wouldn't waste one more *minute* on marriage counseling until he is able to get healthy. You can't help him through it, Lass. I know you want to be able to, to not have the last 3 years and Ian's family be a sham..but he needs intensive therapy and SOON. I feel for him and have no doubt that he feels remorse, shame, and crushing self loathing. Is he willing to go to a 12 step type program? That might add some much needed accountability to his life.
Good luck and I'll miss you. Stop in from time to time, if you can.