I don't think that they way your R and M began is the demise of your M.
My W and I had the opposite beginning..it was all passion from the beginning and we M'd very soon after we met. Passion like that, if you had had it; fades in a short while anyway and then you're left to maintain the M. My W isn't into mundane life and boredom.
As you said, growing into loving each other is exactly what should've happened in my M. IMO, it's exactly how you would want your R to progress. I think your H is using that as an excuse. I just wonder if he also has an aversion to the mundane life and needs the excitement to make him feel alive or to mask inner pain and confusion.
On going dark...I don't know if that's totally necessary if the OW is out of the picture and he no longer actively in an A. If you can get to a place where you're comfortable conversing with him and trust that he's not in an A; I suggest that you do not be dark. Heck, you just went to a counseling session; you're going to need to communicate to get positive results.
