Karlsson's final 2 salary years are $9 million and $7.5 million respectively. A 50% retention money involved is actually $7,177,500 over two years ($3,915,000 + $3,262,500). So the hard dollars involved are less.
Theoretically the Penguins could approach this as a cap dump that would free money used on a position redundancy with Letang and on a team heading to another missed playoffs. Reallocate the cap savings to retool around other roster positions.
Or a 3-way trade scenario as I've posited before with C-Bus having $19 million cap space with the Gaudreau exemption. They could absorb the 50% cap retention while applying the cap space difference between Karlsson's real dollars salary and $11.5 annual cap hit. In what is projected to be a big, high annual growth with the cap, a prospective budget team with C-Bus could apply the cap difference to their own need to keep within the league parameters.
There's a reasonable business case for all three teams involved. On the Oilers front, the Kings game reinforces a real valuable role Karlsson would play on this playoff team. He could be deployed as a second puck transportation option/decoy to give McDavid more room as the primary carrier moving into stacked neutral zone and offensive blue line aggressive PK deployment. I'd moved Karlsson into left flank PP1 replacing Nugent Hopkins to give a right-shot shooter option and elite game processor who would force PK deployment off their overload on Bouchard at top of umbrella.
Then there's the most important piece of an EV elite puck mover for quick, efficient and lethal zone exit work to Draisaitl's Line 2 and Oiler support scoring lines. His skill would help to blunt hard, aggressive forecheck strategies against the Oilers with threat of quick strike puck movement by Karlsson to start attacks the other way.
Karlsson would be a significant X-factor within these tight, hard ice playoff series hockey. And if they wanted to in critical situation play, still manage deployment with Nurse Kulak to offset any defensive concerns.
I don't think the cost would be as high as your proposal. But I would move aggressively to discuss a Karlsson move.