That’s very Rousseauian of you, spoke, but I don’t buy it simply because of the savagery that humans are capable of to each other. IMHO, humans start off Hobbesian but through social conditioning are turned Rousseauian.
I’m not even sure if Hobbesian and Rousseauian are words, but they work. 
As for the whole morality and God thing…it depends on who you ask. There are essentially two main schools of thought on this (at least in the Xian tradition) - the Thomistic tradition and the other one which I can’t remember the name of. The latter posits that the relationship between humans and God is like that of children and parents. Which means that parents don’t necessarily have to follow the same rules that the children do, do to their greater age and whatnot. For instance, little Johnny has to go to bed at 7 even though his folks get to stay up. Essentially, then, human morality is something that we have to follow because God said so, while God doesn’t necessarily have to follow it. Morality is not universal.
Then there’s Thomistic approach, which borrows heavily from Aristotle. This is much more complicated. Basically, morality comes from God because everything comes from God. Those universal “do nots” like murder, theft, etc. are in the realm of natural law, meaning they are ingrained into everyone at birth no matter their religion. No revelation necessary, although, it sometimes helps to remind people at times. Now then, these natural laws are not above God, as nothing is, but rather, are reflections of God’s nature. So it’s not so much as God handing out commandments as it is a reflection of God’s being. Very metaphysical. It’s basically the opinion that the Catholic Church decided it was going to lean towards. I’m not going to go into the nitty gritty of this whole thing, so if you want more info, go read Aquinas’s Summa Theologica.
So yeah, there are your two basic options, both have been in vogue at various times, with Thomas’s being the preferred for the past couple hundred years. But neither one has been set down infallibly as the absolute correct one, so there’s still some debate on the matter.
Lastly, Eonwe, I’m not sure Plato came up with that exact question, especially since he didn’t seem to bother too much with Xian or Jewish philosophy as they weren’t really known that well in Greece at the time.
But the answer to the questino is no, God cannot create a rock so big he couldn’t lift it or whatever other related questions you can come up with. That would be logically impossible, and God is only omnipotent within the realm of the logically possible.