A very short reply to a very long post
But I would search the forum for posts on NRE. It just sounds like M and N are in the midst of new relationship energy and are having trouble maintaining balance with their current relationships. Maybe reading a bit about it might help you (and him) to see what's going on.
The fact is NRE can be great, but it needs to be something he (and she) are aware of, so that they do the things they need to do to maintain their already established relationships. It might take conscious effort to do that, but it needs to be done. They have to realize that they can't just do whatever they want to do or everything they feel like at every particular moment because there are other people involved.
It seems that H is in the same boat as you, so my guess is both of them need to rein it in a bit. Not give it up, and not NOT enjoy it, but be caring of the other people they are involved with.
I'd also tell him directly that he needs to not take what you told him about how you're feeling and turn it into something it's not. If he has questions about what you're feeling and thinking he needs to ask you, not make a wild stab in the dark.