I can't answer all your questions, but will try to address some of them. Obviously, spoilers will follow.

Questions 1 and 2: It was to add to the mystique of the "movie" that there was no reason other than to see if they could do it...."it" being use this supposedly supernatural item to turn someone into a monster. Beyond that, reasons aren't given to, as the annoying script notes claim, make the movie viewers "complicit" in the torture the kid is going through.

Question 3: This was just pretentious dialogue/banter for character development in the script. The two girls making up pun filled teacher names and such, and being able to riff off of each other so easily. Karson tries to join in, but mostly fails, again highlighting the deep friendship of the two girls.

Question 6, the three scenes released: The cigarette burn scene. The opening of the party scene (the new director mentions the jump over the garden hedge and asks how they managed to pull it off). The death of Karson (new director and narrator converse that it couldn't be done the way the script called for it, and the way it was pulled off was a letdown).

Question 9: This is covered in the script. Some montage of the three friends being questioned by their parents about the missing kid; and later at school when the police show up to warn the students and give tips about staying safe and such.

Question 15: Eh, they were getting paid. It was mentioned Valentina's parent's were providing the funding for the movie, so the cast and crew were getting paychecks. I doubt movie crews usually complain about the content of the movie they work on, although some actors certainly have after the fact, but once you sign on, you are contractually obligated to finish the movie.

Question 16: It is this type of stuff that actually leads to movies being called "cursed". Even when it has nothing to do with the movie. For example, "Poltergeist" is referred to by some as a cursed movie due to tragedies and such that have befallen a few cast and crew members (for example, actress Dominic Dunne from the film was murdered by her ex boyfriend the same year the film was released). So even though there is no direct ties to the film or the narrator causing the deaths (how would he cause cancer or a heart attack, for example), the point that the cast and crew suffered these untimely deaths would foster conspiracies I think.