What if Dipper died? Gravity Falls AU

What if Bill Cipher wasn't outsmarted in Sock Opera? What if he was able to go through with his plan to kill Dipper after he obtained the journal?

Mabel's Story: It's been ten years since Dipper committed suicide during her puppet show. She misses her brother dearly, but her life has continued. She has gone to college, she's even gotten engaged! Her life is exactly as she always dreamed it to be...but when the nights are dark and eerily quiet, her mind can't help but wander to the happy boy she had grown up with, and the note she found in her Grunkle's car on the drive to the hospital.

Dipper's Story: Dipper's memory starts about 10 years ago when he woke up inside of a vacant theater. Since then the theater had lain dormant for years, but recently it opened up again giving the restless youth a form of entertainment...besides dreaming to see the outside world. His only friend (and the only person who can see/talk to him) is a sock puppet enhabiting his right arm named Bill. Dipper loves Bill's company, but he hates how he'll never let him go farther than the lobby. He has good reasons though. Bill says that Dipper will cease to exist if he leaves the confines of the establishment. Dipper, however, has been seeing a girl in his dreams that he feels needs his help. A girl he feels that he used to know...A girl named Mabel.