What else could the champions have done to scare up some clues?
Occult research. Is there any history of zombie activity in the area? No, which might point to their magical origin, offering a “what” clue.
The magical source of the zombies might prompt the champions to check with their contacts to see if anyone had recently purchased a grimoire of zombie-raising rituals, which might lead to Bonefat’s purchase of the enchanted goblet. This might be a “who” clue, or if the champions have already deduced the villain, it might count as a “why” clue, used to explain the whole scheme.
Since the slow zombies only last for a few hours, the champions might get a look at the remains. Forensics and a “Detective!” roll could offer some insights. Certainly, a “what” clue, to differentiate magically-raised zombies from, say, restless dead disturbed by poor grave maintenance.
Investigating the identities of the zombies might offer a “who” clue, after realizing that the dishonored dead were all old enemies and rivals of Bonefat’s.
No doubt Miss Dinkley could whip up all manner of useful gadgets that could expose clues. If the champions make enough “when” and “where” deductions, they might be able to set a trap or set up an ambush, eliminating the need for a “who” deduction.
There’s a whole world of clue opened up by tracking. As it happens, none of our champions have the Tracking skill… but they do have a mastiff. In my experience, when a PC pays for a dog as an Ally, that dog is usually half Lassie, half lion, and half bloodhound. Picking up a scent shouldn’t be beyond its capabilities. Tracking zombies back to the cemetery would likely be trivial, but what if the champions took it a step further? It’s not inconceivable that, from the cemetery, they might be able to pick up Bonefat’s trail, particularly if they’ve deduced the timing of the attacks and set themselves to interfere with the next occurrence. That could lead them to the villain’s country house, eliminating the need for a “where” deduction and offering a big bonus to the “who” question. Questioning the servants might yield many more “who” and “why” clues.