Satou and Shio's names (meaning "sugar" and "salt" respectively), seem to be contrasting names. Shio's name especially may seem to not make much sense when the whole of the manga has Satou referring to her as making her own life "sweeter". However, salt is considered a flavor enhancer, and is often used in a number of recipes, including sweets and desserts, to enhance the flavors of them. This is exactly what Shio does for Satou. Brings out the sweet feelings in her that nothing and no one else has been able to.
Speaking of names, the two main male characters have paired names as well. Taiyou can mean "sun" while Asahi can mean "morning sunlight".
This association with the sun connects them with Shio as well. Shio is constantly associated with the symbol of "the moon" especially with "lunacy". The moon and the sun are often portrayed as opposites, however, the moon shines when touched by the light of the sun. Two of the people to whom she glows the brightest are people associated with the sun.
Many times in the series Satou has flashbacks to her childhood that involve her aunt teaching her about what is and isnt love. Then we find out just what kind ofperson heraunt is. We can only pray those lessons about "love" never got physical.
How will Shouko's parents react to her death? We know that Asahi doesn't take that greatly, but he's not her sibling or parent after all.
Given how Shio instinctively goes into her room when Satou kills Shouko, how many times had Shio witnessed Satou getting violently angry?
The final part leaves several disturbing implications. Shio is not only alive, but she either believes that she is Satou reborn or is developing mannerisms not too dissimilar from her. One can only ask if Shio]] would ever recover from her experiences, and if not, will she live Satou's life down to every detail climaxing in her kidnapping a pink-haired girl who she believed to be Satou's reincarnation?
As the Extra Life chapters reveal, no. Although since she has a habit of visiting the apartment Satou ordered the burning of and would bring a Replacement Goldfish teddy bear and a cupcake to there, it's clear that she's no longer in the right place of mind, although it's unlikely she will do anything weird for the time being.
Even if they had managed to get on the plane, it's unlikely that Satou would've been believed dead as she'd hoped. Given that the police apparently identified Shouko's body as they announced her death on the news. Meaning that Satou's plan to fake her own death would have failed anyway as, since her body went undisturbed as is in the chair she was left in, there would've still been proof of the body being Shouko's. Even if her aunt had also tried to take the fall of murdering Shouko, there would be the question of where Satou was, given she was her legal guardian. Satou's plan to vanish off the face of the earth was doomed to failure no matter what.
Not necessarily. The police correctly identifying the body may have depended on the fact they had an extra body to account for. If Satou was the only missing person from the fire (also assuming Asashi didn't go to the apartment either), would the police simply close the case saying the body must be Satou?
Also police is so incompetent in the setting so we shouldn't really be relying on them. An insane woman can put one out of action and things like rape crimes committed by Yuuna's husband go completely unnoticed because nobody bothered to report, after all.
Satou's Aunt may have fully admitted to burning down her apartment and she postponed the investigation for at least eight years (based on Shio being 16 in the Extra Life chapters), but chances are even if the criminal procedures really go through, she's not going to prison. Her psyche would ensure that the crime was a case of insanity rather than pure malevolence, and the police are not competent in this setting. The most likely outcome would be her going back to a "normal" life with a probation officer and/or regular psychiatric visits to the doctor. And unless they have strong enough wills, they too will fall under Auntie's spell...
Satou's obsession with Shio, while creepy and amoral, is decidedly non-sexual and she adores the "purity" of their "love". Shio apparently reciprocating this affection can be justified by being eight years old and not fully understanding the depths of Satou's depravity and their situation. However, if Satou had eventually managed to get away with faking her death and got to care for Shio how she intended, how would she deal with Shio eventually going through adolescence, where she may develop feelings for others beyond their relationship, sexual or otherwise? How would Sato eventually deal with a potential rejection?