LearningToFly,
I told. I would tell again. It, to me, is one of the best things I did after DDay. It gives a second set of eyes on the 'couple' and I felt he had a right to know. This is my story.
Your story needs to be what you want. It's interesting that you said it was a 'rule'. There are no rules about infidelity. It is a breaking of the rules, so there can't be rules, right?
If you believe he has a right to know, and you feel strongly about this, you should tell. If you believe it will help this EA end, you should tell. If you are worried that your H will leave, well, that's another story. My H at first said he would leave, but I wouldn't back down. He did.
Is there another reason, beyond protecting his AP, that your WH doesn't want you to tell? I'm with some other posters here - all of the OBSs are unstable, violent, will take everything from the AP, etc. Why? Because, just like your husband said dreadful things about you, the AP did the same. These disgusting comments about the innocent and unsuspecting BS are made to justify the A. If the BS is a wonderful loving person, then wouldn't the WS need to be a scum to cheat? Yes, so they tell the AP how awful the BS is. It's very standard, and it hurts, but just as your husband told lies about you, you can bet so did the OW.
If your MC is saying she won't see you if you tell, then to me she's just blackmailing you. How much can she 'get' you if you are hurting because of this, and she's threatening you? I'm not going to say that all MCs are against telling, but many are, and I don't know why. Telling is usually a non-issue. You tell, and that's it. Now both of the cheaters are on notice, and both are being watched, so if the A continues there is a much greater chance of it being discovered. Why an MC usually thinks this is a bad thing I don't know, but that's the standard line. Also, telling the OBS is in no way not focusing on your own marriage. It takes about an hour at best, particularly as this is long distance. It's an email, or a phone call. It is hardly going to make you lose focus.
If you feel strongly about it for you, then you tell him. If you are doing it because you are worried about the perception of others, then you shouldn't worry about it. This is about you and your marriage and what you feel is the right thing to do, not what a bunch of people on the internet think. Please, keep that in mind.
In the end, I told because I thought he absolutely had a right to know. I told because I knew if he had been the one to discover the A, I would want him to contact me. I would be hurt and further humiliated that everyone involved knew but me. I would have felt like he would be helping my WH get away with cheating by keeping his secret. I wanted someone on the other side that could make sure that she was also not still involved, because until I told, she continued to message my H. Having the OBS know helps, a lot.
It is your choice - not your husbands and not your MCs. If your H is going to leave you if you tell the OBS, then how much does the marriage mean to him? If it means less than the worry about OW, then he's not very invested in the marriage, is he? As has been said, he's clearly not remorseful. His loyalty is with OW, not you - if he leaves because you tell.
LTF, please, do what your heart says. No one else is living your life. You are. You have to live it for the rest of your life, so if you feel strongly about this, please don't let your WH or MC force you into something that is against what you feel is right.