~Yawn.
You have to make something up for players going the other way or being lost to time, in this case it was Monahan, Dach, Mailloux, Savard, Anderson, Armia, Gallagher. I'm sure some others would come in.
They'd have to, because I don't see a very powerful roster from what you proposed, even with Pettersson on board. It sounds like a top-heavy lineup lacking true depth to win anything and it even still lacks the star power of other teams that can't win anything while lacking skilled depth.
You also had Monahan in your proposed aftermath of the Pettersson trade lineup.
Assuming Monahan, Dach, Mailloux, Savard and Anderson were gone, with Hughes miraculously (and happily) offloading both Armia and Gallagher, I still think you could present a stronger, more realistic lineup, before adding any others that would come in.
Up front, barring anything else coming in, you'd have to rely on prospect panning out, maybe Roy to play higher up in the lineup and add some hockey sense and two-way moxie?
Caufield - Pettersson - Roy
Slafkovsky - Suzuki - Newhook
RHP - Beck - Heineman
Kidney - Dvorak - Ylonen
I still think Montreal would need to make a trade or dip into the UFA pool to upgrade the top-6 in order to truly contend in the future.
However, on D, even with Mailloux and Savard gone, the Habs should remain strong.
However, as much as I'd like to see Guhle - Reinbacher as a future 1st pairing, Mailloux being gone tends to skew that portrait up, IMO.
It doesn't mean that Guhle-Reinbacher wouldn't be put out there, depending on game situations where the opponent needed be shut down, but, a more realistic D-Corps might look as follows, at least until Hutson proved he could handle the full time rigours of the NHL:
Matheson - Reinbacher
Guhle - Engstrom
Hutson - Xhekaj
The Matheson - Reinbacher pairing would be more offensive, IMO and the Guhle - Engstrom pairing would still have the shut down potential lost from separating Guhle from Reinbacher at even strength while the game is not on the line.
Xhekaj would certainly protect the more diminutive Hutson and playing the latter against weaker opposition would definitely provide unexpected/unprecedented offensive production from a 3rd pairing.
Of course, Hutson would also have a PP role on the point, perhaps with Reinbacher as his pairing partner then?
Caufield - Pettersson - Roy (That remains a big question mark)
Slafkovsky - Suzuki - Newhook (that remains a big question mark)
RHP - Beck - Heineman
Kidney - Dvorak - Ylonen (no clue how this line really ends up, TBH)
Matheson - Reinbacher (Reinbacher good complement to let Matheson loose)
Guhle - Engstrom (Good shut down pairing with offensive upside, nonetheless)
Hutson - Xhekaj (Good way to shelter Hutson at even strength)
I'd definitely have interest for such a lineup in Hab Land, but I still think we'd be short upfront to become a perennial contender. However, holes could likely refilled through the trade of futures at that pointer via the UFA front.