First, I'm an engineer, so hopefully I will give you some insight.
You're right that engineers make up their mind about something and then they go to do it. In your H's mind, everything right now are things you have done. He probably doesn't think he's a contributor to the M problems. Therefore, he's decided that in order to get rid of these problems, he has to get rid of you.
This is where you can start working - stop doing those things that brought you to this unhappy place. Nagging, yelling, telling SD what to do, all of those things that might be problem areas for him. Start doing a 180 NOW.
It's my opinion as an engineer that we don't fix things that aren't broken. If they are working and functioning as they should, we don't go looking for problems with them. We won't re-engineer the wheel.
If you start doing 180's he'll see that slowly you're "fixed" and he might be inclined to resume some form of relationship, friendship at first, maybe M later down the road.
The thing is, engineers don't question themselves at any point after they've made a decision unless they see clear indications that they've made a bad decision. Then they'll explore that alternative decision with caution, still thinking they're original was the right one.
Just some experience from someone who is an engineer, and my father was an engineer too. We're VERY logical thinkers. If something logical is thrown our way (hey - she's nice, fun, likes being around me...) we will entertain it.
You have one good thing going for you here - you're in the same house. Steady DB will keep you there.
----"Et tu, Brute?"---- me:28 W:24 S4 T:6 M:4 EA Exposed: 5/21/10 Bomb: 6/20/10 (Father's Day) NC w/ OM: 7/10/10 W moved out 8/21/10 http://bit.ly/aOrZne - My sitch