They might as well swap Smith for Gag & see what happens. My sense is that almost anyone can be plugged into the Bergeron-Marchand pairing and it will work nicely. (Minus Loui last year, for various reasons.)
As presently constituted, this is not a Cup contending club. Sans moves, they will probably finish third in the division, which doesn't bother me.
The defense can be shored up, but what was once a strength is now a glaring weakness without Boychuck. And again, top 3 or 4 D don't grow on trees, and they're seldom traded, even by bad clubs.
As you point out, Adam is not a top three guy. If the Bruins are depending on that, or Bart to play top four, they're done. I'm sorry to harp on this but trading Boychuck was the exact wrong move, even if you can bring in solid or even very good help on the wing. The team is weaker because of this and I don't really see how they address that weakness.
You simply can't win in the playoffs without excellent, solid depth on defense. It doesn't happen, certainly not in the current era.
HIW ("Hope I'm wrong"), but I see an early playoff exit in a division that's gotten stronger while the B's are in "Park" or have even regressed.