You can continue the werewolf chain without completing the assassins/crate quest, I had already completed the chain by the time i went back and finished bloodthorn assassins. Most the quests in the area follow a loose story the bloodthorn assassins quest is just one of the first of them and it ends when you get the skill point (and the castle icon on the map turns white), you don't need a connecting quest to continue the story though even if there is one (don't remember one myself)