To me it sounds like you and DH could update your agreements if they are no longer serving you. Could do this BEFORE you start dating M. It's a good time -- new year, update agreements!
Once that is done, and you want to think about dating M...
I want to see M. I see the improvements he has made and don't see how any of the remaining concerns can get alleviated without giving it a try and seeing what comes of it.
Well, what are his concerns then?
The only one listed so far is drama. If his main objection is that he doesn't think M can stay low drama and you feel M has improved to a place where you can... can DH be ok with you trying it on for X months to find out? Or not?
Is DH willing to behave like a polite metamour or not?
Does DH have any other reasonable requirements of you and M before you start dating for X months? Labs? Meet in person? What?
And if he's not ok and is giving mixed messages, will he clarify? Gritted teeth (action) implies NOT ok with it, and "I'm ok" (verbal) implies yes. Which is it? You could thank him for the feedback and let him know you are going to take him at his word. So is this his final answer? That he is ok with it? Or not?
Because once you begin, it is not fair to be all Mr Grumpy Pants at you because "I didn't really believe you would." If you take him at his word, does he plan to be Mr Grumpy Pants at you? What's his behavior going to be toward you if he feels yucky? He going to talk to you about it or act out? You'd like to know in advance what behavior to expect and hold him accountable to.
Just my 2 cents,