OK, I do appreciate that you trust the source of the information, although I must stress that unless you heard it directly from the W or were witness to such, then it still is third part information. Still, fair enough.
On your specific morals and values, again fair enough. It is more important to have made a choice than to judge a choice right or wrong. Just be sure that you realize your H may have chosen a different path (well, HAS) and although he COULD change his mind and return to a common path, he may not. You don't have to decide right now if that would be a deal breaker for you, and until you DO, understand that you are accepting his current behaviour.
IF your M is saved, you MAY have a problem with his returning to the M. He MAY NOT repent in regards to the As in a way that you are hoping for, if at all. You might have to be OK with that and a possibility that he may also not have learned anything through the process. Sorry to say that. Things COULD turn out the way you hope, as always though, don't expect it. Rather accept it if you can, or set your boundary and enforce it.
While your H could be MLC, there is generally triggers. What you described earlier regarding past issues in his life regarding mother, etc... that's not a trigger, that might be what he'd try to resolve if he did end up MLC, though. A trigger could be anything such as a problem in business or employment, a death which could be something seemingly as benign as the death of a pet or a favourite sport hero, maybe an age thing, the birth of a child... it can be things we might not think would trigger them, but THEY are traumatised by it. Again, is there anything that might have preceded his changes that seemed significant to him?
Even if it is not MLC, there is a possibility that he is simply attracted to the "sin" of the couple in question, rather than a MLC. Things like that can be attractive for some people. Power, influence, taboo, or even just curiosity can be motivators for someone to be attracted to others. What do you think might motivate your H to be attracted to the couple?
It sounds like a complaint like controlling would be a valid complaint. Understand that you may not agree, but if his pattern is holding true then TO HIM it is valid. Could you address that in some way as a 180?
You are justified to not trust him, yet again it is likely a valid complaint for him. Again, how could you address this as a 180 that works within your morals?
Also, considering what I believe is your current context, I would strongly recommend you look into speaking with a DB coach at your earliest convenience. You may decide that you do not want it, but at least inquire into it.
And again, remember that you need to ensure that right now, you are open and honest about your goings on. The effects of your A with his brother is not going to go away quickly and easily. Even if he claims he's "over it", it's unlikely he is. As you mention, his learned behaviour from his upbringing is to hold grudges. How can you 180 or ensure that your course forward is an open book?
