The Devil is not a rival. He exists because it is God's will that he exists. If you would argue that he is "neutral" by perpetuating evil and allowing it to exist, then so be it. But first, let's examine the entirety of the Biblical quote you used:
"I am the Lord and there is no other, there is no God besides me. It is I who arm you, though you know me not, so that toward the rising and the setting of the Sun men may know that there is no one besides me. I am the Lord there is no other; I form the light and create the darkness, I make well-being and create woe. I, the Lord, do all these things. Let justice descend, O heavens, like dew from above, like gentle rain let teh skies drop it down. Let the earth open and salvation bud forth; let justice also spring up! I, the Lord, have created this." (Isaiah 45: 5-8)
As you can see, it's almost comical how the author of the "theory" you propose would take such a quote to disprove the monotheistic qualities of God, yet God actually "addresses" the point in the previous sentence. Yet another Biblical quote taken out of context.
The basic will of God is well known throughout the world. Words like "good", "loving", "saving", and "forgiving" come to mind, but what most people leave out is "just". Justice is one of the ultimate facets of God that people so willingly ignore. Yes, God is caring and loving, but he is also just. I think it's obvious that he has power over evil. Satan is not his rival, nor his equal. If you wish to say so, he can even be called an "instrument of God" as he is even under His power. Yes, God allows evil to exist. Why does he do this? To tell you the truth, I believe it falls under the premise of justice. "Free will". We're given a choice to follow a distinct and set path in life. The choice is yours. God gives us the choice. He allows evil to exist because it is an instrument that people may use in exercising their free will.
If you want a more information on this, I'll post it later, but I'm tired of typing right now.