1) >Crystal Dragon
If you can't catch the reference, that's okay too!
2) As far as I recall, DMC never really elaborated on the broader lore regarding demons where it didn't involve Sparda, and each game seemingly had yet another anecdote to add to his story. Demons exist in a Demon World that exists parallel to ours, and while breaches seem to be a frequent-enough phenomenon, these are almost always small and only allow for the traffic of "anchors" (ex. Trish) or lesser spawn breeds. The exceptions seem to be the grander rituals perpetrated from the Human World, such as that of Arius (DMC2) or Arkham (DMC3).
Beyond that, there's no official explanation, unless maybe it's in one of the artbooks (...probably for the 3rd game, :/ ) or the novels. I've seen some fanfic writers go w/ a more "demonic" interpretation of mythology as having been the original scheme of the Demon World, until Mundus entered the picture and defeated the previous Devil Lord to take its place; that bit about Mundus is canon, but it's practically the only clue given to us about a demonic backstory.
From the games, we know that many Demons are "pureblood", at least in the sense that they come about naturally, or however that applies to Demons. At the same time, we know they can artificially create new types of Demons, though the perpetrator needs to be a Devil of some strength, or else a human dabbler (case in point: Arius and Agnus). Then there's the whole thing w/ defeated Devils usually becoming Devil Arms (barring games 1 and 2), and that raises questions as to why it even happens at all, especially what it means if it occurs in the Demon World proper (and there's no real evidence why it couldn't be the case).
Even aside from Sparda and his kin, we're shown in DMC2 (both novel and game) that Demons /can/ coexist w/ humans, since the Vie du Maril are supposedly demi-fiends themselves, and in the novel the elite Demons and boss Devils from DMC1 appear in an in-universe AU where they served alongside Sparda (and then that verse's *Vergil*) in rebelling against Mundus. So if anything, aside from the "survival of the fittest" theme where the weak are meant to serve the strong, it seems that Mundus is the real enabler of attacks on humanity insofar as wanton conquest goes; after all, he was only sealed away again, not killed, so technically he is still the EMperor of the Demon World and its denizens must still obey him.