So what are the rules? (spoilers but they won't make any damn sense)

It seemed to me that everyone who came back had a death that was related to/caused by Victor. Victor was in front of the bus, and that made the driver swerve and go over the cliff, and Camille kinda caused that as well as him. Peter died because he robbed Victor's house with his friend, and then his friend shot him when they were hiding. They said that Simon killed himself by jumping in front of a car, but that seemed really out of character, and I assumed that later on in the series they would reveal that Victor was in the middle of the road or something and Simon was trying to save him. I'm not really sure about Helen, Lucy, and Adam (the murderer guy); Helen said that she knew Victor before she died, so maybe that's it. And Tony killed Adam after he tried to kill Julie, and that can't be a coincidence. Lucy wouldn't have died if Adam hadn't come back. I'm not really sure why Victor would return though.

I can't really explain anything about the not sleeping or getting drunk thing. Simon didn't stop time in the end, he had a vision of the future. I don't think that was because he was one of the ones that came back; Lucy was told by the people who died in the first flood that they had been sending signs that something was going to happen, and I feel like that was just another one of the signs. I have a theory that each/some of the returned has a power related to their life before/how they died; Lucy lied about being able to talk to dead people, but now she can. I'm not 100% about Camille, she is/was a twin, and she could sense Lena and all that, and maybe her returning amplified that somehow? Not sure how that would explain the guy she killed. That might've just been a bad mushroom trip for all we know. We don't really know a lot about the pasts of the others, so I'm not really sure about them.

Because Julie has the scars, she didn't die. If I remember correctly, at some point she asks Victor if she's "like him". After Adam stabbed her, Tony stopped him, and he wasn't able to finish killing her. I'm assuming Tony either anonymously called 911 to come help her, or someone found her. It seems like a lot of people go through the tunnel (Adam did manage to find like 8 women walking through the tunnel and killed them).

From my understanding, the returned "wake up" where they died (Adam woke up in the woods, Camille woke up where the bus crashed) and Lucy died in the hospital bed, and returned like a minute later, right where she died. Then again, when Simon died the second time, he returned in the morgue, so that doesn't really fit. Maybe it's different when they die again?

I feel like, had the show been given another season, they would've explained all of that more. It is the U.S. version of a French show, so if you want to see if they answer your questions in that one then maybe watch that? I think that one is on season 2 or 3. Apparently that storyline for the first season is almost exactly the same, but I've heard the quality is better. There's also a movie that the series is based off of called They Came back.