Why trade down if Kotkaniemi is who you are targeting? Just take him, don't be an idiot and risk losing him. You should only trade down if the group of players at your pick are inseparable.
If you feel your backup will be there, and you get an offer you can't refuse, you go for it imo. But that's risky and takes being creative and I don't see the Habs doing that. Let's say that NYR wants Zadina badly and offers you 9th, 26th or 28th and 39th.
So you hope Kotkaniemi is there at 9th but if not say you have a good feeling that Dobson will be there.
So you have either,
Kotkaniemi at 3rd
or
Dobson at 9th, Thomas or Dellandrea and say O'Brien or Kaut.
If 1 and 2 go as expected and if NYR told you they wanted Zadina, that leaves 5 players being picked so out of Kotkaniemi, Dobson, Boqvist, Hughes, Tkachuk, Wahlstrom, Bouchard, so you would get to chose between 1 of 2 remaining unless someone else is picked.
Now if they think Kotkaniemi is going to be the best of the bunch or the offer to move down isn't good enough, you just take your guy at 3. But with what I assume is a really close group between 3 and 10 or so you could increase you odds of finding good players by adding some serious value while still having 4 2nd round picks in addition to the 2 extra picks you get.
But it's all a moot point, MB picks Tkachuk cause it's MB.