I agree with you on all points, but Lycanthropy is a curse and a disease. It can be passed on people through a bite, scratch, ingestion of blood and even at birth. Yet it is a supernatural disease and to some it can count as a curse or a blessing. Not to get into a philosophical debate on fiction, but it depends on the afflicted's point of view on whether or not Lycanthropy is a curse or a blessing. Story and lorewise, it can be one or the other. As mentioned, Lycanthropy can be a curse for most, a blessing for some and a way of life for a few. Gameplaywise it is the same deal. Some may hate the drawbacks, some may enjoy the benefits and some simply do not care one way or the other. But in Skyrim, it feels like there are more benefits than anything else. Unbalanced and just plain boring.
This is exactly what I meant and why I brought up the Companions. Aela was proud to be werewolf, Kodlak - not so much in the end.
A disease created by a god is still a disease. It's infectious, you
usually don't choose to receive it, you have to learn to live with it. In Skyrim you don't have to deal with any drawbacks, there's no obstacle to overcome, so lycanthropy feels... bland.
Sinding lost his marbles when he tore a young girl to shreds.
Well, that was caused by a cursed ring and doesn't reflect Sinding's normal behaviour, so I'm not sure if it's a good example. It does show that the wolf is always in him, though.
Alright, I'm not writing another off-topic word.
I don't see why there should be any werewolf castle. Wolves don't live in castles, do they? No reason why werewolves should. However, I'd love it if Dawnguard introduced wild werewolves - that's something I severely missed in Skyrim. If it does, then it's possible that those outlaw lycanthropes decided to form a band and live in a cave somewhere far from civilization.
Still, I think the only new werewolf content will be the perk tree. Dawnguard is all about vampires, after all.