Cazador wouldn't have hired the hunter directly, but through an intermediary via his network of intrigue. I'm not surprised that the order came from the hunter's superior, in fact it seems perfectly logical.
Not necessarily.
When you ask the hunter who this Maiden Fel is, he says she is the reason even monsters have nightmares. This doesn't sound like someone who a vampire lord would hire, even through intermediates. I'd expect him hire someone like the Zhents, who'd deal with a vampire willingly, no need for masquarade.
Astarion is implying the Gur was acting indirectly on Cazador's orders, but he could be trying to misdirect the PC here. He is not willing to confess what exactly he did to the Gur in the past. Maybe he realizes at this point that the Gur could also be after him because of that past, and he doesn't trust the PC with this.
