The two spent casings will be from the rounds that were test fired at the factory. Got 'em with mine too, in a little unsealed manilla envelope. Those aren't anything to be concerned with.
As for the wear on your slide....I haven't a clue. My G23 was new like yours and it didn't have any wear on it at all. Does the gun appear to have been fired? Does it stll have that copper colored packing grease on the inside of the slide?
Sounds as if you have a factory defect, would be my first guess. I would imagine if you send it back to Glock and explain that you got it new that way they should take care of it for you.
Let us know what happens.