With respect to you, you are trying too hard to figure things out that are really difficult, if not impossible, to figure out!
Even trying to figure out 1% of God’s personality or reasoning is enough to fill multiple volumes of speculation. It can literally drive you mad (in the british sense of the word), which is why Jesus recommended that we approach God as a little child, with a simple faith and a gentle acknowledgement that He loves and cares for us as any good parent would.
The role of Satan in our lives can be looked at twofold: First, God still loves Satan, being that He created her (I personally believe that the devil is female and looks very much like Elizabeth Hurley ;)) . God cannot hate what He has created (when the Bible says that God hated Esau, that is not quite a good translation into English. A better translation would have been that God disregarded Esau because of Jacob’s obedience). I think that God would love nothing more than to see Satan repent, but it will never happen because she has completely and totally rejected God in favour of serving only herself, as many people end up doing, as well.
Second, God allows Satan to continue in existence because she serves a function: evil. Evil, though not good in itself, of course, still does serve a good function in that it gives us contrast so that we can understand what is good. If only good things happened to us, then how would we know that it is good? Surprisingly, we actually need evil in our existence not to willfully embrace and enjoy, but rather serve as a measure by which we learn what is good. So until the day God makes things right at the Second Coming, Satan will continue to serve this function, even if she refuses to acknowledge this.
I hope my ramblings have helped a bit! By the way, I love your name! Evanescence ---- isn’t that the same name as that band that has this really sad and depressing song? The video to it has this goth girl all curled up in a ball while children slowly move around her. Great song, but so despairing!