On point four (I have yet to finish reading, by the way, but I wanted to address this while it was fresh on my mind ^_^) - I don't know whether Castiel is inherently male OR whether it is our Western societal predilection to quantify things as masculine, especially when regarding Biblical/religious peoples. I mean, we presume all of the angels, especially the three named in the Bible (Michael, Raphael, and Gabriel), to be male and we bestow such names on our male children. In much the same fashion, God is depicted as male ("the father"), when by all accounts He really should be removed from our narrowed scope of sex/gender and be fully genderless. So, I don't know if Castiel has a gender, or if Jimmy is simply conforming to the patriarchal bias of the Westernized religion.

Or, maybe the writers were being lazy, and after having a male (Misha Collins) playing him so long, they just assumed the masculine pronoun in reference to him.