Here is my theory!
Melanie + Boone are the bathtub babies (Sophia's and Wes.')
Gigi was seeking revenge for her sister Amy's suicide (after the bathroom trauma in 1995.) When it was time to set the plan in motion, she burned Melanie to turn her to the darkside (to become the Red Devil) and also was able to sue the spray tan company for operating money (as she studied law in the Caribbean.) She somehow also found Boone and was able to convince him to help with the killings of the Dean, KKT, and DDS.
When Gigi decided to kill Boone, he survived the stab to the shoulder with Melanie's help (?), so they killed off Gigi and swapped her head for the Kappa turkey. (Boone could still have keys to KKT from joining right before his first fake death.)
Pete is still talking with Boone, who had him help as a red devil at the mall. Pete murdered the police officer (?) and now he feels guilty.
I also theorize that the only people who will survive to Season 2 are the only non-sluts (ie 'Sluts will Die' from Episode 1) Grace Pete Boone (if still alive) Earl Grey (did we actually see him die? He's the only other nice guy) Zayday