Firstly before I post anything else I'd like to write a list of theories I believe.
1. Robbie is a zombie.
2. Grunkle Stan owns book 1.
3. Old Man Mgucket wrote the books and turned psychotic
4. Grunkle Stan suffers from demenita.
5. Mabel owns something special that makes people want her. (Gnomes and Gideon)
6. Mabel suffers from Madness. (Hallucinations, talking to random things, being basically mad)
7. Dipper imagined the whole thing to get away from the pain of his dead sister and abusive uncle.
Anymore? Comment below!