My theory is that POLLY is the goblin king. She wanted Alice and Betty to join the Farm, she is clearly in on the farm's hypnosis scam, and she invented the lie about Jason being the GK. She's crazy enough to join up with her father, the Black Hood. And maybe Hal Cooper was the original Goblin King in his youth, and he's passing the mantel down to Polly?

It would fit in nicely with how Betty is the Griffin Queen in the Griffins and Gargoyles mythology. Also, how Polly is trying to hypnotize Betty into giving into the fact that she’s “a killer, just like dad.”

Jason Blossom's grave is empty because Polly dug him up and has him sitting in a chair in a room somewhere on the farm - That's probably what Cheryl is seeing (under the influence of drugs) when she envisions that Jason is alive.