Question: What was Chucky and Tiffany's childhood like? Because I read that Chucky's back story was that his father was an alcoholic and occasionally beat and raped his mother and would often beat Chucky up, did Tiff have a good childhood?
Answer: The films don't delve into either Chucky or Tiffany's backstory, and the films themselves are the only official canon. So until it is properly addressed in the movies, it's basically up to your imagination.
Question: When Chucky killed Barb onwards, where was Alice? Was she in the house, or did she runaway to her gran's at the end?
Answer: It is revealed when Ian checks the webcam that Chucky hid Alice in the closet. She was most likely there the whole third act.
Answer: When Ian is checking the nanny cam, it is reveals that Alice is playing Hide-And-Seek with Chucky, instead of sleep. Chucky hid Alice in a closet, to keep her is not on his way while he is murdering the family.
Question: Why didn't Nica use the video cam footage to prove innocence?
Answer: It's not any problem and trouble in the film. Maybe it's just a reflection of light in the bed.