I'd say Trelawney is a fraud AND genuine. She obviously doesn't realize she actually has the gift. She thinks she's just pretending, and so does almost everyone else. Most of her predictions have been totally false. She has predicted Harry would die in a hundred different ways, a hundred different times, and it hasn't happened any of those times. Most of her major predictions are only true when she is in trance mode, and she doesn't even know about those predictions.
Her first name is Sybil for a reason. JK Rowling obviously named her after the famous true story of Sybil, the woman with multiple personalities. And we see that Sybil Trelawney's demeanor totally changes when she is in trance mode. The name is very fitting.
But now that you guys have mentioned it, she is oddly accurate even with her "false" predictions. What's interesting is that the Lightning Struck Tower card not only predicted disaster, but in this case, a literal tower.
And yes, Luna's quote made it pretty obvious that Sirius will return. Her quote was something like "The things we lose always come back to us, just not always in the way you expect". So Sirius will probably come back somehow, but in an unexpected, surprising way. He won't just be pulled back through the veil, it's going to be something much more shocking, perhaps he won't even really return to life.