Question: After Miguel comes back to the land of the living, does the family believe that he went to the Land of the Dead? And if not, then do they believe that Hector got murdered, since the letters to Coco prove that he was the writer of De la Cruz's songs? And if they believe, do they manage to prove this to the general public and that is why they hate him? Or do they just hate him because they discovered that he lied about writing his own songs?
Answer: It is never actually addressed what Miguel told his family about his adventure, so it can't be said whether or not they believe him. It would seem to make sense that he would tell them though, it was a pretty fantastic tale after all and I'm sure he would want to tell his family about it. It would also make sense that his family would believe his story, it's really the only way he could have known that Remember Me was a lullaby Hector used to sing to Coco. The letters Coco saved prove that Hector was the actual writer of De la Cruz's songs and it was later proven to the general public, hence the graffiti on De la Cruz's memorial and a memorial for Hector taking it's rightful place in town. It can also be reasonably assumed that the general public is aware that De la Cruz murdered Hector since the theft of the music and Hector's death go hand in hand. It wouldn't make much sense for people to hate De la Cruz so much if all they thought he did was take credit for his partner's music after his partner was already dead.
Question: If Ernesto stole Hector's guitar, why wasn't he cursed too?
Answer: The criteria for being cursed were never accurately established in the film. All we have is the words of the specialist: "You're cursed. [...] Dia de Los Muertos is the day of giving to the dead! You stole from the dead!" This seems to imply that the act of theft must have taken place on Dia de Los Muertos and the owner of the stolen object must have been dead.
Answer: In my opinion, I would say it may have been specified in Hector's will that his guitar would go to Ernesto in the event of early death.
Question: Could someone please identify the character voiced by John Ratzenberger?
Answer: Juan Ortodoncia - he's the one who gets to cross over to the Land of the Living because he's remembered by his dentist. It seems like he also still has braces on his teeth.
Question: Why didn't Miguel become cursed when he stole the chicken leg off the grave offering? Is that when Dante became cursed, since technically he ate the chicken leg so he stole it? Or is Dante just a spirit guide the whole time?
Answer: The chicken leg didn't belong to a dead person. Its legal owner was alive and well. The spirit of dead do not take possession of the offering. They take a spiritual copy of it.The legal owner of the guitar, however, was dead.
Answer: Miguel became cursed not just because he stole an offering, but because he stole from his family. Dante is always a spirit guide.
Answer: Clearly intent has something to do with it. When Miguel took the chicken leg, he wasn't intentionally "stealing" it. When he took the guitar, he knows he's stealing it. His intention is to borrow it, but he knows what he's doing is wrong.
Question: What chord does Miguel strum in De La Cruz's mausoleum after he steals his guitar?
Answer: G.
Question: Why doesn't Miguel fall through the bridge like Hector? Miguel doesn't have a picture on the table stand.
Answer: Miguel isn't dead yet. He's slowly fading away the more he stays in the land of the dead and will eventually be stuck there just like everyone else. Until he fades away he can still walk on the bridge but he needs a blessing from a family member in the land of the dead to actually get back.
While yes he is not dead, the rest of the movie becomes entirely pointless since this means he could have just walked back. Security wouldn't have stopped him because he is alive so they know that it is urgent for him to cross the bridge.
He can't just walk back. He can walk on the bridge but he can't actually get back to the land of the living unless he gets a blessing from a family member.
It was Miguel taking the guitar that put him in his dead/not dead state, not crossing the bridge. Miguel was already only able to be seen and interact with the dead while they were all still in the cemetery on the living side of the bridge. It stands to reason then that crossing back to where he started wouldn't help.
Question: What discovery does Miguel make while in the sinkhole with Hector?
Answer: That Hector is his great-great-grandfather, not Ernesto de la Cruz.
Answer: It is never really explained in the movie, but we could assume that whenever someone dies they keep whatever items they had on them. Hector may have had the photo on his person when he died.