Question: Why did the king and queen disappear when the king signed the contract? The deal was that Fiona will be saved from the curse if he did - but Fiona wasn't saved at all. Was the contract void from something?
Question: In the beginning, the king and queen are shown riding in a carriage. However, before watching this movie, I viewed Shrek 3 where the king had turned into a frog and died. If he died in 3, how did he come back to life in this movie? This was before the king and queen signed any contract and vanished, so magic was not involved in his being alive.
Answer: It was stated in this scene that Fiona was rescued from the tower, meaning this scene took place at the same time as Shrek 1, when the King was still alive.
But they didn't have kids in the first?
Question: What happened then to Prince Charming or Fairy Godmother? In Shrek 2, they did mention that it was the Fairy Godmother who cursed Fiona. So how come Prince Charming never rescued her instead of Shrek (since he was never born in Rumpelstiltskin's timeline)? The King and Queen may have disappeared but at least Prince Charming could have still saved her.
Answer: It is not known why it took Charming so long to rescue Fiona. She was in the tower for several years. Shrek 2 picked up after she was rescued and it was too late for Charming. Since Shrek was never born and Charming was so late, I would think there was nothing from stopping the King and Queen from signing the deal with Rumplestiltskin. The deal was for Fiona to be rescued from the tower in exchange for the Kingdom. He never said "who" rescued her. She rescued herself. It was a trick from the beginning. Not sure of their whereabouts after the deal. Fairy Godmother probably could not change the alternate reality if she did not know what happened to everyone.
Answer: Although they never say explicitly what happened, my guess is in the alternate reality where Shrek never came to rescue Fiona Prince Charming was killed by the dragon (like all the other knights who tried to save her) and then Fiona escaped on her own shortly afterwards.
Question: How does erasing the day Shrek was born change anything that erasing any other day of his lifetime wouldn't change? After all, it's not like he just suddenly came into existence that very day. He already existed in his mama ogre's belly. The only true day to erase that would have the intended consequences would be the day he was conceived. Without that day, the exact "tadpole" that would become Shrek would have a low chance of winning the same "race to the finish line" that allowed Shrek to be.
Answer: First of all, it's nebulous magical rules in a fairy-tale setting. It's easy enough in the context of a magical spell to make the leap that "Getting rid of the day Shrek was born" = "Shrek doesn't exist." (For all we know, he simply vanished from his mother's body that day.) Second... do you SERIOUSLY expect them to make a kids movie where they discuss a baby ogre being conceived sexually, hahaha? Like... really? You know this is a movie that small children watch, right?
Answer: Rumpelstiltskin used a contract that 'made all their problems disappear'. He didn't reference any specific problem and the king and queen assumed it would cure the curse. However, it just makes the king and queen disappear, and since they have disappeared, they will no longer have any problems.