Duuude, DEAD ON for this. I also agree with most your points but this was one of my biggest disappointments with 4. I became convinced after finishing DMC4 that Capcom just don't want to do anything with the lore. Not that engaging storylines is a pillar of DMC, I know it's just a backdrop. But it's just insane to introduce a new playable character, then say NOTHING about his origins. Hell, Lucia got a ******* origin!
And hell, implying he is may be the son of Vergil actually makes it worse, no clarification. "What? How? WHEN?"
Thanks. Yeah, I mean, if they'd been a bit more clear on Dante's age (since we know he and Vergil are twins, and all) then it may have been easier to digest Nero being his son. I can see him wanting to pass on the Sparda legacy, even if he didn't want to be a parent, himself. I can see it being a, "In the event I die, someone can carry father's torch, because I love/hate my brother, and don't think he's capable of doing the job, himself" or something.
They just...never really gave us more than a vague idea, all around. What they gave us left us wanting more. I can't tell you how many times I've searched online, and there's a lot of conjecture about this, that, and the next thing-but very little confirmation. Just a tad frustrating, lol. >.<