I think PlusGas is more powerful than WD-40. I'm too poor to have experience of Chris King headsets, but again you shouldn't have actual serious rust, even though the cheap fork has a steel steerer and it is a good fit for the steel bearing cases. Again I would soak in PlusGas for a few days. Full force hammering with your right hand can't really harm the frame provided you hold the top tube with your left. Don't clamp the frame though! Your left hand is the damping mechanism.