My sense of Eight is that the caring is real, but that he can stop caring in an instant if sufficiently disappointed or angry (the way he turns against Karl in YOIT, for example).

Right, yes. He's a lot more... fickle than most people make him out to be. But also a bit more devious.

I'm just glad you don't hate him.

I don't think I could actively hate anyone remotely involved with the Doctor Who series. I'm that hooked. :D