Well, this is an Au fic after all, and the whole point of one is to do something new and different.  I don;t think I have demonized, as you say, James, Sirius, or Dumbledore, merely pulled them off of their pedastals and shown them to be mere mortals.  In canon, the way James and Sirius behaved towards Snape was injustified and shameful, they were bullies, plain and simple, though eventually, God only knows how or why, they grew up . . .somewhat.  Obviously enough so that Severus was willing to have them babysit his son.  As for Dumbledore, the man in canon was shown to be manipulative in the extreme, leaving Harry with the Dursleys who were horrible and abusive for the sake of "protection".  And putting a teenager in charge of defeating a mad,am when he was supposedly the most powerful wizard ever and knew that Voldemort feared him.  To me that is negligence, he gave Harry no real adult guidance, merely hints here and there of his grand scheme.  As for Severus, here he is given the chance at a regular life before he becomes a spy, and by not making him a Death Eater, is not angelicizing him at all, it is sinply makeing him choose a better path.  By allowing Lily to defend him and stay by him, he takes the other road, the road he would have taken if Lily had been faithful to him in canon.  JKR even says in an interview that Severus would never have become a Death Eater if Lily had asked him not to.  So, in my fic, she does and he's not.  Beyond that, however, he;s still sarcastic and grumpy and sharp-tongued, he still has the temper and resents Harry up until the moment he discovers he's his son, so to me that is not Severus Snape the angel, it's Severus with an epiphany. 

But to each his own.