Why Big Mom wanted Uta before she knew she was Shanks' daughter. Because she has lived long enough to recognize her devil fruit and its power to control people, it could be used on giants which would give her the power she wanted from Lola's arranged marriage.
Why Uta tells Luffy that Shanks destroyed Elegia despite the fact she is the one who did it? There is two possibilities:
Uta still sees Luffy as her friend and secretly hates herself for destroying Elegia. She didn't want Luffy to see her as a monster.
The ending of the movie might initially seem weird. Uta is dead, and Luffy just jumps up and says he's gonna be the Pirate King again. But when you think about it, it makes sense. Luffy remembers Uta before saying it. He's inheriting her will for a peaceful future, thereby reaffirming his desire to become Pirate King.
Two reasons for Uta becoming so huge in two years from the start of her career to the events of Red.
First is that her devil fruit allows her to be a one woman musical composition team. She's her own special effects team, her own sound mixer, her own makeup department. While her fruit is affecting you, and that is everyone who she's interacting with via video performances, she can do it all. So she was able to amplify her own skill and talent with music with essentially a budgetless effects budget without any time restrictions.
The second is that she debuted at a period of time with increased tensions all throughout the world. She was in a very good place to be a escape for the citizens. This very need to escape the world also would have led to influencing her own plans as it became very organic for her to find people who wanted to escape the world and go somewhere better.
Why was Rob Lucci in Mariejois when he is typically the Cipher Pol agent of choice? Two reasons.
The first is that killing the entrapped civilians was not the first plan of action by the World Government. Lucci's infamous 'Dark Justice' would make him very likely to default to that instead of stopping Uta, if not just go outright killing the citizenry after killing Uta.
The second is part of the fridge horror of Uta winning, but if she did win it would pay to have Lucci to deal with the resulting pandemonium. No reason to risk one of their best assets should things go pear shaped instead of having him ready to deal with said catastrophe.
After Uta showed that she was more in synch with Luffy than expected of a pop diva with her chicken game, it is easy to catch a second parallel between the two. At Sabaody Luffy punched Charlos in the face for a (good) reason and that caused things for the Straw Hats to go down the toilet (leading to the crew being separated for two years). Uta meanwhile trapped Charlos for a (good) reason, which in turn was the point that some of her fans started to want to go home which really started to send her already bad mental state to further go down the toilet.
Why did Sanji throw away a mushroom at the begining of the concert? He knew it was poisonous, foreshadowing how dangerous something Sanji threw away was.
Uta's bounty of 15 billion berries, almost ten million higher than Gol D. Roger's. While it was primarily used in merch as a play on the net profit the film made, it could easily be fathomed that the Elder Stars slapped such an obscene bounty on her after realizing that Uta just put the lives of 70% of the world's population at risk.
Uta discovered she is the one who killed everyone in Elegia and Shanks lied to her for her own good. It may be why she was not listening to everyone when they told her they want to leave. As far as she is concerned she is protecting everyone from the dangers of the world and is fine with being hated for it.
Uta's song when she summons Tot Musica contains lyrics that are shown in an Elder Futhark/Runic language. The scene itself that goes with the song, when everyone on Elegia witnesses Tot Musica's reawakening, emphasizes just how dangerous the song is in the wrong hands, how evil the monster truly is, and how far off the slippery slope Uta has gone when she resorts to this.
For that matter, the note sheets of that song appear very discolored yet preserved, to show it is an evil and forbidden artifact of sorts.
On the subject of Sanity Slippage, all of Uta's songs save for the last twonote "The World's Continuation" and "Where the Wind Blows" when she finally gets back to normal, courtesy of ADO, actually reflect her mental state throughout the movie. "New Genesis" represents her optimism at the start in hopes of achieving peace. "I'm Invincible" represents her confidence against pirates who are after her. "Blacklight" shows how she's not going let anyone mess with her goals as she captures Luffy's friends. "Fleeting Lullaby" emphasizes how desperate she is becoming, but now knows not everything is as bright as she thought thanks to the Marines. "Tot Musica" is when she completely loses all hope and just wants to destroy everything. It's especially frightening as her songs can warp the reality of her dream world, for better or worse.
With Uta now dead, that means the Sing-Sing Devil Fruit will sprout again. And once that happens, let's just hope they don't use this fruit for their own intent purpose of killing people in their sleep.
Fortunately, in a morbid sort of way The Sing-Sing fruit probably reincarnated somewhere safe as long as either the Straw Hats or the Red-haired Pirates had a single fruit, which the Straw Hats certainly do. The Straw Hats having such a fruit isn't an old theory either, the Snow-Snow fruit reincarnating into one of Nami's tangerines has floated around for a while. That being said, the fruit being in their hands will be a bitter reminder of the dead Uta.
Had Uta won, even without Tot Musica, the people who she'd have affected would have been in a bad place, but the 30% who would not have been affected are in their own uniquely screwed pickle.
First is that of the people whose souls were not trapped in Uta World forever, they would have to deal with a world where over half of the worlds population suddenly dies. Economies across the world would have cratered, to say nothing of all the various necessary services like food production, non-Marine security, and political leadership Society itself would quickly start to unravel, with even the Marines unlikely being able to keep order everywhere even if nothing else happens. Very crucially, among Uta's fans shown in the end credits are the top brass of Galley-La, meaning that should Uta have succeeded a shipyard so crucial to the World Government their often uncooperative leader is unkillable would be thrown into disarray, a military bottleneck that would only exacerbate the following problem.
Second is that other things would certainly happen as of the three groups who had any idea what Uta's power would do, two of them were Emperor's crews. While Shanks would certainly not choose that moment to embrace grand anarchy and take over the world, Big Mom is another matter altogether. She might have lost Oven and Brulee in this scenario, while the Marines might not have lost comparable elite forces, but she would still have the core of her crew, including Karakuri and Smoothie and most likely Cracker. Nothing is stopping Big Mom from launching an all out attack on the Marines who, unlike with Whitebeard and Marineford, would not know where she would attack or when. Big Mom would have full control of where she would make her move, and the results would certainly be bad for world peace.
Third is that this only includes the ones known for certain to escape Uta's plan. The Marines would have to deal with chaos all over the world, including their own ongoing conflicts with the former Warlords, that would certainly come to include the forces of Kaido, Blackbeard, and The Revolutionary Army if they avoided Uta's powers, and with the sheer chaos and surviving discontent there would be fruitful ground for all three of these forces to act. While the Revolutionaries would certainly be the least destructive and most moral of all the involved parties, this doesn't mean that Uta's actions would almost certainly lead to at least one Marineford level fight between the Marines and an Emperor, possibly multiple Emperors and the Revolutionary Army, in a war whose chaotic effects may be impossible to compare to anything short of the Void Century itself. While some other parties would certainly be taken out by Uta's plan, as among her fans seen in the credits included the leaders of the Firetank Pirates and Kuja Tribe, it would not take many enemies of the World Government slipping by her performance to make life horrible for the remaining thirty percent.
