I don't think i follow. Why would Kat need to make up a story about being blackmailed, when she could've simply told Edgar instead "the guy just suddenly vanished and we don't know why or where he went". It'd be just as effective cover-up.
And yeah, if the guy had any brains at all then obviously his blackmail would involve a threat that if something happens to him then whatever he's got to blackmail with wouldn't just vanish. I mean, he'd be asking to just get shot in the head on the spot the moment he brought the whole thing up, otherwise.
On the other hand, and to play devil's advocate for a moment, we get these Chekhov's Clips, so to speak, on August's board:
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The one about the skull is dated June 7th. I doubt they're accidental.