I don't think that Shashara was "playing by the rules". She and Vasher had knowledge that the rest of the world will probably never know. Prior to Vasher, it took insane amounts of breath to create lifeless, instead of the way it's done now. Also, Vasher created an ARMY of awakened lifeless soldiers encased in stone which shouldn't be possible even with the wealth of breath given to the god-kings. My guess is that they found a way to awaken steel without using nearly so much breath. It's possible that they found a way to power their awakening with another type of investiture, since they had been off-world. What if they had been to Roshar and found a way to make a fabrial which transferred investiture directly to them as breath from stormlight? Sounds crazy, but who knows? But I am reasonably certain that there's no reason to believe that she would have had to hold as much breath as Peacegiver to be able to awaken nightblood.