No, you aren't understanding.
Kostin has no trade value I agree, but he could be moved imo with a pick for a "older" veteran that could help. Cap dump basically with a pick. Carters just my quick example it's not like a favorite move.
So you ship Kostin and the pick for Carter and the rest of this year, plus dump the cap on Koston in 24/25. Now it depends on how much Penguins like the pick for the cap dump.
Fair enough, I misunderstood your last trade proposal. Considering what Carter has done this year, it'd have to be a different player though. Maybe someone like Tyler Johnson who's won a Cup or Zucker who's a more useful player. Mantha would be hilarious for multiple reasons, but Edmunson would be a more likely trade candidate with the Caps.
The more I think about Mantha, the more I like the idea. Every Wings forum on the web will explode, and he should be motivated since he's UFA, plus Larkin seemed to like him.
Edit: Plus Raz could give him the mean face to motivate him, because we all know Mantha only plays well when pissed. II also looked at his playoff stats, and I don't think any other team will get into a bidding war for him.