For Karlsson to work, Pittsburgh either has to retain half and take back a player with term like Perron or Amadio. Zub would be another candidate if they are worried about his durability.
With that said, moving out the Zub contract because of injury issues would be ironic considering they'd be doing it to acquire Karlsson who with his age and injury history would be at risk of also missing games. They would also get worse doing that because they wouldn't have anyone to replace Zub's role. I'm certainly not advocating that trade.
If they are looking to move Norris, they could trade Norris for Karlsson (with some retention), or trade Norris to a third teamas a move to facilitate acquiring Karlsson. A Norris trade to a third team likely sees a lesser bad contract coming back to subsidize his cap hit for the acquiring team, and then Pittsburgh taking that incoming bad contract from Ottawa and retaining to subsidize Karlsson. I only bring up Norris, because his name has been in trade rumours over multiple seasons. I assume that there won't be a market to move out the cap hit of Norris without taking back negative value cap or retaining. Which is why a loose 3-way deal for Karlsson might make sense.
Karlsson is not that unrealistic. It's not on the same tier as bringing up JT Miller or other big stars. He is very clearly available for trade. He has ties to Ottawa, so he might waive his NMC. We also have a similar big negative value contract (Norris) that has been in trade rumours over the past two seasons. So it isn't as if the proposal is take 10M x 3Y and move out zero cap. I do think expecting Pittsburgh to retain half is unrealistic, but they may have to do something close to that to move him if the market doesn't materialize. The timing to make that kind of move does not feel right with the team rallying for a playoff spot.