Some of the objects that were made into the vessels of sin are pretty obvious in why they were chosen (such as a sword for Lust due to the obvious comparison to a certain body part, four mirrors for Pride due to the Narcissist implications of needing more than one, etc), but it's not really as obvious how scissors play into the sin of Envy. Until you take into account how they're used, particularly in Kayo's song: rending something apart so it can be re-purposed for someone else.
A yellow/green/red/blue four-color motif shows up briefly in "Handbeat Clocktower"—it doesn't seem important at first, and perhaps a bit odd because the rest of the song isn't color-coded, but makes more sense upon a revelation that the song is an arrangement of "Heartbeat Clocktower," which is also known as Clockwork Lullaby 4. The colors are also a reference to the four Story of Evil novels.
In the "Tailor of Enbizaka" PV, due to multiple artists working on the piece Kayo's inherited scissors are drawn inconsistently as both eastern scissors and western scissors. At first it's the same fiasco as the Venom Sword being drawn as a rapier, but its actually pretty clever considering in Heavenly Yard we learn there really were two pairs.
On Project 'Ma', there are two overly flowery lines near the end of Adam's part that seem a bit of a non-sequitur, but when you look at it taking Adam and Eve's Meaningful Names (with Zvezda meaning "star" in Russian), it becomes far more clear "The Evening Stars were corrupted/The moon and stars disappeared in the southern sky" refers to Eve's mental instability thanks to the brainwashing drug, and her and Adam eloping to Elphegort, south of Levianta, respectively.
As noted by more than one fan, Pride in Judeo-Christian theology is the strongest of all the sins and considered the source of the other six; given that, it's natural that the Daughter of Evil is by far the largest arc in the franchise.
While they are all represented by Miku, look at the hair of for example Eve, Mikaela or Mikulia and then look at the Master of the Court, the Doll. The Master of the Court's hair is pretty much always definitely blue, while the others have teal hair. Why? Because the Master of the Court has not only the Doll's powers by the point that we get to see her, but also those of Gear, that is, the Marlon Spoon. Remember, that same character was drawn with teal hair back when she was just Gallerian's daughter...
Sloth not having an actual Demon of Sin makes sense when you realize that it is usually known as a passive sin by nature that isn't directly responsible for evil deeds, but causes people to grow apathetic enough to allow evil deeds to happen.
It seems a bit odd that "Five Pierrot" would be the answer song for Sloth, seeing as Lemy and Margarita seemingly had no connection to each other beyond both of them being members of Pere Noel. But then, recall that Lemy is a reincarnation of Hansel and Margarita is actually Eve Moonlit, meaning that their connection runs far deeper than initially thought.
Nemesis declaring herself "Master of the Hellish Yard" and singing the song representing that character despite there already being an existing character with that title actually makes sense when you consider the fact that Nemesis is also Levia, who was the original Master of the Hellish Yard.
"Lulila the Resounding Song" and "Chrono Story" seems to mix up the chronology of the sins, putting Envy before Sloth. This may, however, have an in-universe justification now, due to the World's End arc: Michelle Marlon replaced Margarita as the most compatible Sloth contractor, she was born after the Envy arc and died during the Greed arc.
Eve isn't just a Genesis reference—she and all of her incarnations are references to famous Fairy Tales, and in some cases, refutations of them:
Eve herself is a beautiful woman who had doomed herself by taking apples from a witch. She is Snow White.
Mikulia is a poor girl who is mistreated by her family until a wealthy "prince" sweeps her off her feet. She is Cinderella.
Platonic is a thief who steals from the rich and well-off in order to help the poor and needy. She is Robin Hood.
Margarita is a maiden who finds herself in an eternal sleep in part thanks to a wicked old woman. She is Sleeping Beauty.
In terms of them being refutations, Eve dooms herself of her own volition rather than being tricked and indeed, steals from the witch rather than being given the apples; Mikulia's desire for a happy ending blinds her to the obvious danger of falling for a man who she hasn't gotten the chance to know first; Platonic ultimately found herself unsatisfied with her life as a thief and returned to the life of leisure, embracing a lifestyle she once scorned; and Margarita, again, enters eternal sleep on her own rather than being cursed or tricked, with the wicked old woman (Irina/Julia) only providing her the means of doing so.
The story Seven Crimes and Punishments reveals that after the different afterlives merged, some of Venomania's victims chose to enter a sexual relationship with him. But it's also said that many others did not join them... What are their thoughts on this whole scenario, having spent centuries in the afterlife presumably happy before they're suddenly thrown into another world, one where their rapist can contact them again? Especially disturbing is what Carol and Lilien in particular must be thinking, given that their dear friends Gumina and Lukana are among the women who went back to Venomania and yet they don't appear to be present at all.
In the Master of the Heavenly Yard novel, it's revealed that Michelle made a contract with Eve, and using her powers caused her to look, think and even act like her for a time. Let's hope for her sake that she never slipped into her "Eve" mindset while being around her father Gallerian, who looks like Eve's lover Adam...
Since Flower of the Plateau!Mikulia was actually the Clockworker's Doll, one has to wonder about the logistics of her husband trying to sleep with her. Hope he didn't get splinters down there.
mothy makes plenty of references to the real world, but sometimes these references cause confusion about how they came about in-series. An example is Riliane's last name as "d'Autriche"; out-of-universe, it's a clear reference to Marie Antoinette, but in real life Marie was called "d'Autriche" because it's French for "from Austria". In-series, though there is no Austria in Bolganio...Similarly, the Levin Church carries crosses but have no Jesus figure who died on one to venerate. Even with the explanation that Evillious was, in-universe, modeled after Earth, it still doesn't explain how some of these oddities developed.