The Seven Sins appeared in the human world before the tower was raised, and they can do so given that they have a medium to appear as (in their case, sand). They spawn everywhere without the Temen-ni-Gru— in fact, they were what bound the tower in the first place. Being unsealed a year before in the manga allowed them to go around wreaking havoc. For an in-game example, they show up in Mission 1 while the tower is raised after Mission 2. If this is the case, why and how does Lady get away with saying that she and Dante "destroyed the remaining devils and that was it" after rendering the tower inert? They should still be spawning since they are never resealed.
They were likely there purely under Arkham/Vergil's orders. With both of them defeated, the Sins retreat, and Dante and Lady are able to kill any remaining stragglers.
If sealing the tower required such a complex ritual in the first place, and unsealing it took so much time, what happens to it after the demon world is closed at the end of the game? Does it just... stand there?
The Seven Sins resurfaced in the present time, in what's assumed to be Modern America. Is there any reason why they suddenly know Japanese when Vergil offers them the chance to rename themselves?
This Troper thought that the Sins were living in Japan until the Tower summoned them. Or were serious Japanaphiles.
How did Vergil manage to avoid all of the Gatekeepers of the Temen-ni-Gru while Dante had to fight them?
Is that so hard to guess?
Cerberus, Agni and Rudra: Vergil was already at the top of the tower when it raised, so he didn't have to go through them.
Leviathan's Heart: He never got eaten by Leviathan. I don't think Leviathan was a Gate Keeper, though.
Nevan and Beowulf: Vergil took a more direct approach to the lowest part of the tower (where the seal was), probably thanks to Arkham, so he didn't have to pass by the rooms of those two.
Gergyon (or however that horse was called) lacks an explanation. Then again, we never saw where that bridge leads to, anyway.
Doppleganger: Like with Lady, Dante had gone first though there, so the path was open.
There was an extended Mission 13 scene that featured Lady actually holding her own against Dante and Vergil before the plot twist was revealed. The scene was cut short because "Lady can't last that long against two half-demons". Cue what actually happens, then fast forward to Mission 16. Lady, after having her leg impaled, after climbing up the side of a really long tower, and still tired, initiates a boss fight with Dante.
Most players tend to be able to curbstomp Lady at that point though, so YMMV.
Also Lady's boss fight mostly involves her running away and spamming projectiles, whereas in Mission 13 it would have been a relatively close range fight with no cover or higher ground to take advantage of, unlike in the library.
Mission 3: Why does Lady need to see the Temen-ni-Gru through binoculars? Not only is she situated in the alleyway that's just a few blocks away from it in the first place, that thing is SKY HIGH. It shouldn't take much effort to find a tower that dwarfs every other building in the vicinity.
This one I can answer. She was using binoculars to look at the top of the tower, where Arkham was at the moment. Her line next second? "I found him".
The line was "I found it". It, not him.
Fridge Logic I would say. Wouldn't you say "it" to your father after he killed your mother, possibly beat you up, and is doing everything in his power to be a demon (or at least, half-demon), and take over both the demon world and human world?
Lady repeatedly refers to Arkham as a 'he' (see the line below about "undo[ing] what he's done", and the following lines "He was obssessed with becoming a devil, so much he killed his own wife. For that he butchered innocent people too. He's the most vile kind of creature!" and "I'm driven by the inability to forgive him! My soul is screaming, demanding me to kill him!") If she was trying to distance herself from Arkham by referring to him as an 'it', she did it horribly as that's only said once.
Just what kind of sacrifice was the Priestess involved in? Any time Arkham mentions her, she's referred to as the whole being, not just her blood ( "[...] in whose body flows the same blood as the sacrificed woman [...]" "Even Sparda had to sacrifice a woman so that he could become a legend!" etc. etc.) Assuming that the Priestess didn't just prick her finger and call it a day— since that wouldn't be much of a sacrifice— she either bled a lot, or bled to death. Which of the two would it be, and wouldn't the unsealing ritual require that much?
13th Avenue (and the area around the tower) is empty. All the humans are either dead or evacuated, so who was calling Dante's phone?
The Sins (or at least just the Prides) were released already in the manga. The manga takes place up to a year before the events of the game. Everyone was either dead or evacuated in the space between, sooner rather than later.
So why would Dante set up his new shop in an abandoned city?
"He's my father. Besides, who else can undo what he's done?" Uh, hello Lady, do you just happen to believe in No Ontological Inertia? Killing Arkham doesn't exactly undo the fact that he took the lives of innocent people unless there's a ritual involving trading off his soul for someone else's that's never been heard of.
She might have been talking about her own inner turmoil and hatred, as in, "what he's done" to her, anyone else killing Arkham wouldn't give her the closure and revenge she was there for.
How is Arkham able to appear as Jester in one of the bonus fights when he's already in hell? Is the fight supposed to be non-canon or is that Jester somehow separate from Arkham? At that point, what's the point? Dante already knows who he is.
Why does Jester need Dante to "Destroy the lock" for him? The door in question just leads back to where Dante already was 5 seconds ago, and Jester leaves via moonwalking on the ceiling. Further, we later discover that Arkham and Jester are one and the same, thus he should already have access to all areas of the tower (or at least the sections he actually cares about.) As a side note, Jester tells Dante that violence is not the answer to opening the door, then tricks Dante into opening the door by hitting the console: with violence. What?
This one has been bugging me for years. In between Missions 10 and 11, when Arkham and Vergil unlock the large door to "The Lair of Judgement", Vergil chastises him for failing to kill Lady and stabs him before namedropping this trope. After that, Dante finds his "corpse" and there is another shootout with Lady. This cutscene is set in another, much smaller, room with a much smaller door with 3 symblols in a circle (the one that will lead to a different room each time the tower spins). How did Arkham get there while being severely wounded? If he crawled all the way, where are the bloodstains? Even if he did go full demon already at that point, there's no way he could shrug off frickin' Yamato so easily, so he could wait for Vergil to enter the Lair of Judgement, start getting busy with the ritual, then just stand up, walk away without being seen by Vergil, then get to another room and play dead so Lady could find him and he could spill his bullshit in her ears. And there is no indication that Arkham posesses the power to create doppelgangers, so there is no implication Arkham Vergil stabbed wasn't "real" Arkham.