And yes, how Snape could possibly have learned Occlumency well enough to fool Voldemort always puzzled me, too. Some older fanfic has Dumbledore cultivating him as a spy for ages, tutoring him in it and preparing him for the role, but now we know that didn't happen. He had very little warning, in fact, and turned relatively late. And yet, he must have been good at it before the Potters' death. Hmmm.
Snape is a character who begs for more backstory, IMO. There are so many years left blank, in canon, when complicated things were clearly going on.