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.
Corrected entry: Ernesto de la Cruz was exposed in the afterlife as a fraud, but the movie went on as if he was also exposed in the current present life too. The boy could have mentioned it, but would he really have been taken serious on the subject there? Ernesto would have gone on and been idolized in the real world, unfortunately.
Correction: It's mentioned in the movie that Coco's letters from her father contained all of the songs Ernesto put forth as his own, proving he stole the songs.
I'm sorry. That wouldn't prove anything to anyone other than Coco and maybe the family directly.
Nevertheless, that's the explanation given in the film. It's been a year. More than enough time for them to investigate the letters.
To add a bit more to the above explanation... Ernesto de la Cruz would likely have fabricated a back story to support his claim of authorship of the songs, which the letters to CoCo would have refuted, leading to scrutiny of the author.
Question: When Miguel first meets Hector, Hector claims he used to work with Ernesto, who Miguel initially thought was his great-great grandfather. When Hector confronts Ernesto after Miguel first meets him, Hector reveals how Ernesto murdered him with poisoned wine. Why doesn't Hector tell Miguel that earlier when they first meet?
Answer: Because Hector didn't realise he had been poisoned until The scene where they meet. It is only after he sees the scene from Ernesto's movie that he puts it together.
Exactly. It's shown in the film that Hector, and other souls, believe he died from eating bad food. Hence his embarrassing nickname.
Question: What happened to Ernesto after he was crushed by the bell in the Land of the Dead?
Answer: It's been confirmed that he is still alive, so he'll continue to exist in the afterlife for as long as people remember him in the living world. And considering the "legacy" he left behind, I'd say he's going to remain in the public eye for a very, very long time.
The legacy was actually Hector's and everyone in the living world found out and decided to forget him.
Answer: I believe he died the final death because his skeleton body was presumably crushed. Hope this helps.
Also, a sign with "Forget" has been put on Ernesto's tomb in the living world, which would speed his demise if no-one remembered him.
I think he would suffer a worse fate of being held under the great weight of the bell until he is "forgotten" in the living world. Seeing as people hate him now in the dead world they may leave him there.
Answer: He was trapped inside the bell. He was not crushed. He was probably able to get out but, the family was able to escape before he got out. Also someone probably needed to help get him out.
Ernesto is a murderer and when everyone found out, they hated him. There is absolutely no point in helping him.
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.
BaconIsMyBFF
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.
BaconIsMyBFF
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.