Pretty sure Barron doesn't fetch a late first round pick in a trade today (he was selected at 25th OA), but Harris could perhaps fetch a late 2nd round pick (he was selected 71st in the 3rd round).
Weren't getting a top-6 player forefather of those Ds, IMO but, an eventual trade of a D, along with draft picks,or as part of a bigger package, might well helps acquire a genuine top-6 player.
At that point, I don't see the worry about making room for other young players currently in the system?
If the acquired player makes a case for himself and is better than Heineman, Mesar, Roy, or whomever, what's the problem with slotting him higher up in the lineup?
How about a 3rd line of Mesar-Beck-Roy? Or Newhook-Beck-Roy?
A second line of Slafkovsky -Dach-Mesar?
This year's draft pick(s) in the first round (if Monahan returns a 1st round pick), or Montreal's own first round picked a prospect acquired in a trade involving Monahan, could seriously impact Montreal's future top-6.
Would you trade Monahan + Matheson (50%) for Holtz + Devil's first round pick?
If Montreal somehow added Holtz and one of Lindstrom, or ...-Nygard, plus Iginla, we'd have a good chance of seeing two of those players making our top-6, adding a scoring touch and dropping players like Newhook and Roy to the 3rd line, and players like Heineman to the 4th line.
I'm not sure where Mesar would fit, honestly? 4th line with Heineman and F.Xhekaj at C at first?