I also got the impression that he has the knowledge of a way, not the means. Yeah, Hemo-gobbler certainly will not want to talk to any of us. The only chances of getting the information from him seem to be making him submit without killing him(unlikely) or sending Stas's new character in (which is dangerous, may be difficult regarding in-character reasons, and generally not the strong point of Stas's style of character).
Also remember that he is an undead wizard, who usually have a particular dark focus in their studies, so the revival of Mystra that he's talking about may not involve so much life. That said, I'm not sure Mart would do that. It certainly isn't outside the realm of possibility. I don't know much about the Cult of the Dragon and I'm not sure how much is known in-game, but it's possible that that is their goal or a step in their plans. Hell, that could have been why they were helping the vamp or he could have found out through them.