As i understood it, Megan doesnt become more human by being reincarnated. Its actually quite the opposite. During her infiltration of the Reckoners, she almost never used her powers. Thus she returned to being "normal" or removing Calamitys negative influence on her. By being reincarnated however, her powers are being used again. That is why she seems so cold to David, I mean a normal person would not act this way.
I think Brandon can pull it of logically. Brandon has explained earlier how he divide his worlds into the cognitive, the physical and the spiritual (?, not sure about the last one) The cognitive being buildt from the idea that every object has some sort of awareness. Therefore intent from humans can influence the physical world.
Thats why i think the gun hurt stealheart, because in the cognitive realm the gun identified the being who acted upon it, and
therefore David and Steelhearts emotions and intent could affect how the physical world plays out. Also, i think thats how the epics powers manifest. The powers exist both in the cognitive and physical realm, They are mostly limitied in the cognitve. Would also explain how weaknesses can seem so random, like Curveballs. His power can only be used from and object that identifies itself as a gun.
Brandon extrapolates on this in The emperors Soul Where the magical power has a similar dilemma.