For your first question, to attain the highest form of meditation, The one who practices meditation must be able to separate his inner atma or celestial body from the physical body through the head to higher points and self. For the disciples, it is difficult to concentrate and elevate with all the hair. So they have less hair on their heads. And for the Swamis, Gurus, Sannyasis etc, It is easy for them to elevate but they keep their hair because it helps them to maintain that elevated state. It acts like a barrier.
For the second question, It is mentioned in that article itself that Himsa and Ahimsa are relative terms and are dependent on circumstances. When Vishwamitra attacked Vashista, Vashista was not the sannyasi who had quit everything in the world, he was more of a spiritual Guru who was working towards the well being of humanity and such. He was not alone, He had to protect his disciples and the holy calf Nandini. So it has nothing to do with "a true Sannyasi should not protect himself even if attacked."