Abby being Wakefield's daughter was part of the theory that I was referring to earlier. The fact that they brought it out as an open possibility with so many episodes left makes me question whether it really is the case. If it's true, then I would have expected it to come out in the last couple of episodes. I'm still thinking it might be true, but if it is, there must some even bigger "reveals" later. Hard to top that, but who knows.
I think that Abby having a half-sibling actually makes sense with the reveal being this early, but who? Assuming it is one of the "listed" characters (and I think it must be), then I can only think of 3 possibilities off the top of my head. The wedding planner, the old boyfriend, and the bartender friend. Yes, the bartender friend is one of the "listed" people and between you and me, I don't really remember seeing her ever, but according to IMDB, she's been in 7 of the 9 episodes...
As for the sheriff, I'm hard-pressed to believe that Madison is actually working with the killer, so I can't think why she would have lied about who took her and she knows who Abby's dad is. However, I do have a theory about that. If the killer is back and the sheriff suspects that either Wakefield is still alive or had an accomplice, then he really wants to catch that person. Maybe he suspects that Wakefield never was the killer, I don't know, but I think he's desperate to catch the guy this time.
He knows this is about Abby and thinks that if Abby leaves the island, the killings will stop and he will have missed his chance to catch the killer, so he "kidnapped" Madison by telling her they were playing a game and had her make the phone call.
If that's the case, then Madison was never truely in danger, but it makes Beth's death even more odd because it means that both the sheriff and the killer are using the tunnels.
BTW, it seems you missed what happened to the stepmom. Yes, there was the pool of blood, but then they panned around to the back of the chair and the pruning shears had been stabbed into the back of the chair. To me, it looked like they were only pushed in far enough to scratch her back, but we got the idea.