I was just thinking yesterday to start a topic on this very thing you bring up, and look, here you have done this already! (These things do not surprise me anymore).
I agree with you, no surprise there either. I wish to expand what you say above with another element, and that is how this relative good and evil correspond with degrees of our relative awareness. Someone who is much more in the traditionalist mode of understanding sees the world in terms of a battle between good and evil. There are friends and enemies. You are either on the side of good, or you are on the side of evil. You either serve good, or you serve evil. You are either 'saved' or you are 'lost'. In or out.
But as our conscious mind moves outward, we begin to see others less as 'outside', and instead see ourselves in them too, they are like us, even though they are 'foreigners' to our ways of thinking and our conventions of how we live our lives. This is a furthering of an expansion of our consciousness, just as moving into the mode of thinking the world is controlled by external forces of good and evil (God and the Devil), is a move of consciousness further out than believing the world is connected to our thoughts and controlled by them (in the way the child sees his shadow as part of himself).
To take this to the parable of the wheat and tares, I believe Jesus through his Wisdom was speaking to the literal, traditionalist minds of his audience who sees the world as in this pitched battle of good versus evil, couching what he realized within those terms they could understand, but was saying something far more profound, far beyond just some battle of good versus evil. He realized that all levels of seeing the world, all modes of thought, coexisted together as part of the system of evolution, or "growth" as he would understand that, towards the greater, or Absolute Good.
The 'tares' are really those imperfections that work together within ourselves, and within our systems, as we move towards that Absolute. You are right, the 'evil' is relative. It is not truly evil in the sense of an absolute. It is simply that which is the stone against which the blade is sharpened. In the highest sense of the word, the "devil" is a servant of God. (Certainly this is true in the OT portrait of Satan). In the end, through refinement, the dross is removed to reveal the purity of the metal. But to remove the fire, to remove the system, to remove the stages of growth, the processes of evolution, would be to leave us unrefined, in slumber, in an unconscious state not Realizing our Unity with God.