Okay for nerd you need to analyse all the evidence from the crime scenes, even the non-related stuff, at the first crime scene (where the body is found) the most common spot people have missed is the stuff that is up the hill near the fence, if you approach it you'll have a QTE to get up there. At Mad Jack's you need to scan everything so while walking towards the back of the workshop press R1 continuously, also go into the office and scan there and scan the acid tank to progress DO NOT i repeat DO NOT approach the exit as this will trigger the fight scene. At the club don't forget to turn off ARI to pick up the 2 fuel receipts on the floor near the fish tank and also don't forget to pick up the killers gun behind the door.
You are able to play chapters through chapter select to change the outcome and there is no need to start a new game. With regards to Ethan I'm narrowing it down to coincidence, the trauma he suffered after the death of Jason would explain why his physcosematic state changes greatly. I'd say him turning up on Carnaby after the blackout's is because we're meant to to think he's related to john and also the killer.
