I see where you are coming from but I think that maybe you misunderstand what oneness refers to.
While the world is multi-faceted, in the sense that we observe it as such, I think it only seems that way to us from our point of view.
If you "zoom out" your perspective, at least for me, it is usually observable that "the many" are actually parts of a bigger "one".
I am sure you are familiar with the concept that good would mean nothing without evil.
How would a completely good or completely evil world look?
How would you know in which one you lived if you had no way of comparing it to something else?
I think that is the point you were trying to make.
But to me, oneness doesn't refer to choosing to do only good or only evil, or having any other 'one way of life' as you put it.
True oneness for me would be a perspective in which both good and evil are unified and you use them both when approaching life.
The way a battery works is having a positive and a negative node that are connected. It is only because of the polarization between them that electricity can flow from one to the other.
Wouldn't you say that having both of the opposing forces work towards the same goal (in this case generating electricity) has unified them?
And that though they are clearly different from each other and "plural", the system that is making use of them for a greater purpose that either couldn't achieve alone is "one"?
By embracing that good and evil are actually working towards the same goal, you can achieve a perspective that is beyond both of them and that can achieve something greater than either good or evil could on their own.
To me, this is the process of spiritual evolution.
If you unify all of the "many" aspects within yourself, you continuously evolve beyond them and gain a better perspective.
If you go beyond all the different aspects of the world and unify absolutely everything, you will gain a perspective beyond the world, and that might as well be synonymous with being God.
So the conclusion is that while the world is composed of many, there are always bigger "ones" in which they can be unified, with the last "one" being God.