MERL has caught most of the jist. The plans you uncovered after beating Lacedys were about him being an alchemist and sending hosts of 'rage' potions to the Mansion Club (not the city). Your invitation, received before the graveyard, also stated that Anatoly, your original quest-giver would be 'attending', with the lead up from the communications book to that invitation its like he's been abducted. Also, with Lacedys knowing so much about the operation, and having escaped your hands, it might be likely he's around as well.
You have no knowledge as of yet how the rage substance will contaminate the drinking supples (not restricted to water), however, behind the mansion club is a river, and near the front door is a well. I've also touched on how the water and beer is supplied troughout the mansions different venues, namely via an intricate tube system that lines along the different ceilings towards the respective bars (though it hasnt been remarked upon). If wine is also included in the poisoning that has to be done via barrels.
Furthermore, indeed, Zalfide is a vampire, and should be undercover here as a pole dancer, with several people under her influence, and more believing her cover.
Considering Phrez, i guess he's most likely to return to the pits on second thought yes. All in a realistic way of course though, so he wont race off before you've finished your breakfast and entered the Mansion again.
Link to the original invitation:
viewtopic.php?p=86936#p86936" target="blank" target="blank
Link to post about Lacedys' correspondence:
viewtopic.php?p=88618#p88618" target="blank" target="blank
(there are some more posts on these letters scattered around when you for instance talk about it with Maryana, slowly revealing it centers around enraging/ causing frenzy via the drinking supplies. As far as you know everything points to magical tampering, though how and exactly what remains a mystery. It would be rather badly thought trough if everything would be okay with a single dispel magic or waiting trough the 1 rnd/lvl duration of the spell with the name. Actually, the original spell is even one that can only be applied on willing creatures)