If this is a bluff, They have blown their single shot.

If this is not a bluff, they owe the world the list of names, no matter what.

So what is the optimal strategy for the cartel? Release the captive and wreak terrible vengeance if Anonymous doesnt hold up their end of the bargain? Call the bluff?

Since the information Anonymous claims to have is probably already known to the authorities or at least suspected, I doubt the cartel fears its release.

The cartel's optimal strategy is probable to kill the hostage in some gruesome and horrible way, as a message to Anonymous to stay out of their business.

Anonymous can attack governments and corporations and individuals because all of those generally have to go through legal channels to fight back. The Mexican drug cartels do not give a damn about legal channels.

Anonymous are like kids that have been throwing rocks and taunting animals at the zoo, and now think they can safely get away with taunting a grizzly bear they've run into in the woods.