God is the greatest possible being to exist. He's that by definition, so if something created God, then the created god is no God at all, because the thing that created god would be God. When you think of it that way, of course there is nothing that could ever have created God.
Your argument is nothing I haven't seen before. Hear me out and try to look at this from an open mindset. You ask, how can God be infinite while the universe can't?
The answer to that is something I consider reasonably simple. The Cosmological argument makes you look for an uncaused cause, so, you ask yourself, what are the things stopping the universe from being the first cause? Well, the universe is physical, so it is bound by space and time. Therefore, the argument causes you to look for a cause that isn't bound by space or time.
Thus, this first cause is timeless, spaceless, and holds quite a bit of power, because it would've had to set this complicated universe in place. Hmm, that sounds lot like the common depiction of God, don't you think? And him just "being" is set apart from the Earth and stars for clear reasons (that is, the problem of space and time).
As to who would've witnessed God creating the Earth, I'll respond to you with the question of how Moses (the most likely writer of Exodus and Genesis) knew that "the Lord hardened Pharaoh’s heart?"
Addressing Cain and Abel, I believe you continue to misunderstand the tale of the two brothers. Allow me spell out the story for you:
Cain made a sacrifice of some grain and Abel sacrificed the best of his flock. God was pleased with Abel's offering but not pleased with Cain's and, as a result, Cain was furious. At that point, God warned Cain that if he did not do what is right, sin would be crouching at the door. Cain then proceeded to murder Abel and God confronted him about it and so on. This is an accurate telling of the story; I used the Bible (and not my memory) to write it and you can fact check it yourself.
Looking at the properly told story, ask yourself, how justified is Cain now? Yes, you could put yourself in his shoes and understand why he was furious and jealous, but does that mean he's justified?