Maybe the NHLPA's best chance is to drive home the point that, not only are the Kovalchuk and Hossa (etc.) contracts the same, but by rejecting one, it leaves it up to the league's discretion whether it wants to meet up with the system arbitrator at a later date to throw out the Hossa deal, and maybe the Luongo deal, too, and maybe Zetterberg and Franzen, etc.. And some very disturbing consequences arise from throwing out those contracts. For example, if the arbitrator rules that Hossa's contract is void, then the commissioner has it entirely within his discretion to declare any and all games Chicago played with Hossa last year retroactively forfeit.
Obviously, Bettman -- unless he actually literally loses his mind -- wouldn't do anything of the sort.
But the NHLPA could argue that ruling against the Kovalchuk contract would lead to de-registering at least some of those other contracts, at which point there would be massive grievance filing from all corners of the league (Detroit, Vancouver, Philadelphia, etc.), possible defections of players whose contracts are de-registered, all of which would be the #1 NHL story for the foreseeable future, which would be catastrophic for the league in this fragile state etc., etc., long argument short: it's simply too destructive to go down that path, for the league, for the players, for everyone. The NHLPA would be arguing, in essence, that the NHLPA and the league are on the same side, the side of preserving the NHL. Yes, they all screwed up when they left themselves open to these contracts, but what's done is done, and luckily there are only a couple more people this could apply to (next summer) before the loophole is closed and the problem is solved.