That may not be a fight the league wants. It's hard to prove he can never play again, since concussions can act strangely. So you get one doctor arguing Pronger can never play again and another doctor that says Pronger may be able to play again and then you probably couldn't force a retirement. Team, NHLPA, and Player all say they want the contract to continue in hopes he can play again, tough for an arbiter to overturn on a he said/she said type argument by the league.
He's 39 at the start of next season. This is the kind of stuff they used in the Kovalchuk arbitration. Let's be honest if it wasn't Comcast, this would be handled differently. He'd be gone of natural causes within a season or two even if he played all the way through.
All NHL contracts have always been guaranteed for players injured during play - what you'd be doing is forcing Pronger to give that up. It's not cap circumvention.
But a 35+ deal is supposed to stay on the books even if the player is retired. At minimum they should be forced to buy him out