I don't...know how vampires work in Diva's world. I barely know enough about my own. Once someone is changed into a servant, though...there's no returning to being human. What's done is... already done. Any other price is too high. [It's even more complicated than that.]
The only path I can see in freeing Conrad-san is killing Diva. [But to make matters worse] I don't know if he requires sustenance from her, or anything like that. But whether he desires to or not if she's his master he'll not have any choice but to stop you from any such attempt on her life.