Well, that would be a convenient way to stitch up the plan, but the hole in it would naturally be how Carter was informed of the date in the first place. I still think that collusion with Zac, or a convenient go-between, is more likely than a happy coincidence. I also think that silencing David was at least as important to him as ensuring the date went forward. Playing the scene again, I also worry that we only have Carter's word that he arrested David instead of dumping his corpse in a ditch somewhere. We never did see him either arrested or imprisoned. He just disappeared.
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TLDR : Carter knew about the date from David and the MC. Carter was hired by Harry to kill the MC and Charlotte but he also had his own agenda.
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I've only read a few posts here so I don't know if it was mentioned, but during the golf event we see Harry talking on the phone, telling someone that he wants Charlotte and the MC "gone" quickly and that the FBI is snooping around. I believe that
someone is Carter. Jason then mentions that the guy is a psychopath. Seeing how Carter killed Zak in public and then followed and tried to kill Emma, it makes no doubt in my mind that he is the mentioned psychopath. Plus, when Carter meets with the MC in front of the hotdog stand, we can see Agent Fox sitting on a bench, spying on them. Finally, Carter freaked out when he heard that the date wouldn't happen, probably thinking it would be his best chance to get Emma without the MC. If there is collusion, I think it's between Harry and Carter.
Zak didn't know who Carter was when they met, he clearly asked, "who the fuck are you? Are you a photographer?" Carter didn't know who Zak was before David met him in his office (he could have been acting, but I don't think he was). Zak was also an idiot who only wanted to get his dick wet for fun and prestige sake. I hardly see him being part of a conspiracy to harm Emma or Charlotte. I don't think he was colluding with Carter in any way. I think the date was just the perfect time for Carter to act and that's why he framed David to make it happen.
Carter knew about the date through David. The exact line is "He's not interested in Emma. He's going to date Emma so he can get into their house." Carter didn't know when it would happen, but he learned it from the MC during the same hotdog stand scene I mentioned before: "Emma was supposed to be going on a date this Friday, but we've had to tell her she can't go."
It's pretty clear though that Johno was taking pictures for him. Carter did a Freudian slip when he said "That fucking idiot!" before catching himself when the MC told him about Johno. But since Harry didn't know who the MC was (we know that from an exchange with Harry and Jason in an underground garage), we can guess that the stalker thing and bringing the MC back to Hillside was, as some of you mentioned, orchestrated by Carter and he almost confirmed it while he was holding a knife at Emma's throat. He had his own vengeance in mind (we don't know why exactly yet but it has to do with the first Emma).
What isn't clear to me, and I don't know if it's a mistake on the writer's part or if it's just because Carter was mentally unstable, is why he killed Zak and planned to kill Emma. When he killed Zak, we saw his inner thought : "Murdered by a jealous love rival, along with his innocent girlfriend." So his original plan was to kill both and frame it on another teenage girl. He had absolutely no reason to do that though
as far as we know. And I can understand why you'd want to stick to the "Zak colluding with Carter" hypothesis which would explain why he'd kill Zak. But all the rest points against it IMO.
But Carter didn't kill Emma right away when he found her, however. He waited for the MC and Charlotte to show up. I think he saw that as a perfect opportunity to get both his revenge and the job he got from Harry (if my hypothesis that Carter was hired by Harry stand). He obviously messed up bad and since the MC killed him, it might take a while until we find the truth, if we ever will.
Edit: I mentioned that Carter didn't have a reason to kill Emma. But I've re-watched the last scene and he might, actually. He wanted the MC, and possibly Charlotte to suffer the loss of Emma. He should have stuck to that plan if he wanted to succeed. Letting the MC and Charlotte catch up was greedy.