Interesting proposal, Monahan and Danault are basically at opposite ends of the center spectrum, one is full attack (what MTL could definitely use), the other is full defense. Not sure what the add from MTL would need to be given the contract situation. If CGY is fine with Danault + Lekhonen + Chiarot (3rd pairing or fringe 2nd pairing Dman able to take a spot in the top 4 if an injury comes up, I'd value him at 2nd in the current context, taking Green as comp), to accomodate the 6.4m from Monahan, I think that could be fine from MTL point of view. However, I have seen on other trade discussions that most CGY fans value him higher, so I don't think there would be an unanimous agreement between fan bases.
The issue is that in my opinion, it does not matter which player you bring on board, you still have Julien & co on the bench.