I don't think it'd be difficult to move any of Zucker, Kapanen, Dumo or Petts. I just think the biggest hurdle is the team actually wanting to. Sully adores Zucker, and there are a ton of stories about how well liked he is in the lockerroom, etc. Teams usually wait way too long on guys like Kap, who will inevitably become the newest in a long line of Galchenyuk-esque nobodies who got a buncha leeway because of their "former 1st rounder" tag. Dumo's living on the memory of his play during the back to backs, and I think a ton of GMs would see him as a safe, proven vet with a ton of playoff/Cup experience. Petts is probably viewed as a safe middle pairing vet with some potential upside left to reach amongst GMs around the league.
I don't think any of these guys are unmovable, nor do I think any would have to have a sweetener attached. Mayyyybe Zucker, but that's it. I still think retaining on Zucker is more than enough to move him for pure cap. But it all goes back to whether or not the team wants to move on from any or all of these guys, and I'm skeptical.