I think Karlsson is a much easier sell in the off-season once the cap is like $10 million higher and teams like Florida and Carolina don't have Ekblad and Burns on their books anymore. The Penguins would still have to retain, but Karlsson at $7 million should be workable for either team with their contracts expiring. The Penguins could take more money back to improve the return, but I think they could at least get something for Karlsson at $7 million once the cap rises.
It's one thing to take back a single bad contract like Dumba from Dallas to make the deal work at the deadline, but if the Penguins have to be saddled with numerous terrible deals to make a deal, they're better off just waiting until the off-season when the cap shoots up and more teams have money.