Politically, in terms of who is ruling each region, it's probably pretty close to the truth. Crusader Kings can be good about that stuff. That said, the way the nobility in any given region interacted is going to vary, and thus CK2's system is probably to broad to be an indication of what happened. Generally speaking, the the aristocracy of Northern Italy came from a number of populations that gradually became enmeshed with each other. There was the old senatorial elite that held on to some of their power, there were Lombards, and eventually there were Carolingians. The relationship between the Empire and the nobility of northern Italy was in a constant state of flux and the degree that any centralized authority could exert of the region was temporary. No game is going to do that kind of complexity justice.

If you want more information, The New Cambridge Medieval History is pretty good for a general narrative. Chris Wickham has a pretty good book on some of the relationships you're talking about, http://www.worldcat.org/title/early-medieval-italy-central-power-and-local-society-400-1000/oclc/7576748. It's quite old, but it's not too bad. The best source for your purposes, however, would be the early Middle Ages volume in the Oxford Short History of Italy series, http://www.worldcat.org/title/italy-in-the-early-middle-ages-476-1000/oclc/48532606&referer=brief_results. It's more recent and has some good scholars.