Sorry, but you're missing some points.
1. Sparda is 2000 years old, he didn't have to have a wife at the same time as Eva, he could've had one before her. Its that simple. May I ask why are you do think that he only had one wife? He could've had multiple women. For example, Lady is the descendent of a priestess who aided Sparda in sealing Temen- ni- Gru. Whose to say that Sparda didn't have a relationship with that priestess? Then there's Lucia and her grandmother Matier, their family had fought alongside Sparda during his rebellion. Is it not possible that he could've had a relationship with any of their women? You give me legit proof that Eva was Sparda's only woman, then your point can be valid. Otherwise, you're just scoffing off a possibility for no reason.
2. How long was Agnus in the order? Not long since he said, "Its only been years I since began this research..." Here he's talking about being able to bind demonic power to various objects. If we assume that he's been in the Order since its beginning, and the Order has had this ambition since its beginning, then his statement would mean that the Order has existed for only a few years, which is not true because the Order has existed before Nero was born. However, if we assume that he had been in the Order since the beginning of his research, then its safe to assume that he probably doesn't know much of its past and any other research it has done. Also, considering that people were "p-p-p-p-privy" to his extistence, I'm going to assume that he wasn't there for long since its impossible for a person to be kept a secret inside an organization since its birth without somebody finding out about him and spreading rumors. The reason is this: assuming that he's been in the Order since its beginning, why hide him? It would be very convenient for Sanctus to just say "here's our new scientist, who would be looking into the workings of the demonic world to help aid us" or something like that. Otherwise, if he hasn't been there for that long, then it would be okay to assume that he doesn't have much info on any other of the Order's experiments. Furthermore, if he was to have info on other experiments and he reads files concerning an experiment on Nero and that file were to say "failed," then it would be logical for him to deduce that Nero doesn't have demonic powers or that they are relatively weak and not much of his concern.
Then there's Sanctus who obviously knew of Nero's demonic powers for a long time. How come he knew, yet Agnus was completely baffled? Even moreso, after Nero beats him down he asks, "That's the demonic power... How can this be?" My question is, if he didn't know of Nero's abilities, then what's this "the" that he's talking about? I doubt that he's talking about demon powers in generaly, so he's obviously talking about something specific.
3. If it was so obvious then this thread is completely pointless. Nero's white hair could just be a genetic trait, possibly one of his parents had white hair without being demon. Besides, if what you say is true, then any trait associated with demons would indicate that that person has demon blood but that's not true. Matier's clan is supposed to be descended from demons, yet she holds no signs of having demonic blood, and Lady has superhuman agility, reflexes, and endurance, as well as having a red eye, yet she's human (a red iris is impossible for a human to get, by the way). Plus Nero's DB isn't his own, he was attacked and seemingly got the DB afterwards: "From that day forth... my arm changed..." Note that he says this after receiving images of Kyrie being hurt and it also states in the DMC4 manual that he got attacked on his right shoulder, indicating it was the attack that had caused the DB to show up, not any of his own blood. The reason being is that Dante and Vergil have been attacked and harmed by demons yet no change has ever been provoked by those attacks, yet they are direct descendents of Sparda with his blood. If the DB was formed by a conglugation of demonic factors, including his own blood, then the same should've been for the twins which would mean that Dante's demonic powers would have surfaced at much earlier time than DMC3. Furthermore, the DB doesn't change in appearance but in power, and it only changes by attaining certain items. If you what you say is true, then the DB should be evolving on its own, but it doesn't. Basically, the formation of the DB has little to do with Nero's blood, his blood being nothing more than a genetic conduit for the transformation to take place, but rather the demonic force that is possessing his arm and allowing it to change. The only implication of it being his own, however, is of the concept art of his DT which is in your sig, however it holds no canonical importance considering that the design was used for the apparition that shows up in his DT, not his own physical transformation.
Basically, give me some concrete proof that you're right, and your argument would be valid. Otherwise, I'd advise you to remember that nothing is set in stone when it comes to Nero. You may think you're right, but there are other possibilities.