Kamon is a perv #1 because he is a guy (I'm a guy, and I will admit that all guys are pervs...but so are girls, not as much but still^^) and #2 he is f'n lonely. You can see that he really drifted from his father already and of course has no intention of returning, and he smothers Naota because of it. Kamon is very insecure, which would give a reason as to why he would take his wife's surname. Kamon is constantly projecting his own feelings onto his son and then wants to rationalize his son's projected behavior. But as we know, this never works!!! Once a parent begins to use a child to solve their problems, most of the relationship is nulled.
Shigekuni took the name probably because of respect, to honor both his son and his daughter-in-law.
I lean more towards maternal death. Naota was obviously hit by it a while ago, as he NEVER talks about it. While a death may plague a person's mind for a while after it happens, no matter how close that person was to you, they will drift away from your thoughts. As a result, Naota has repressed her position in his life, and replaced it with his older brother, whom he looked up to with great respect and admiration. Naota, like his father, is insecure and looks to others for stability.
*WOW that went all over the place, but I'm glad to post here after so long^^


