Why is there something rather than nothing?
I can almost agree with everything you wrote, but there are some things I would say differently. Like the concept of nothing. It's an interesting concept, and like any other negation it pressuposes the thing that it negates, and that means that nothing, by negating something actually affirmes something - something is metaphisically prior to nothing. Nothing cannot "be" without something.
So, there you go. It's simple as that. That is why something has an advantage over nothing.
But yes, as someone already mentioned above, nothing, thought as opposite of something, doesn't exist, and that is why IT DOESN'T EXIST.
The kind of nothing (or should I say nothingness?) that "was" metaphisically prior to nothing-something dualism is actually the One, that which is beyond every conception and every dualism.
So, the One was first, then it split in two: being and non-being. And why that "happend" is another story. I'm not even sure if I was clear enough on what I wanted to say here, but... Oh, well.