That's a new one, but then I found a few books on Abe Lincoln recently that blew my mind too... in one, he was Gay and in the other he was a legendary Vampire hunter.
According to a couple of friends of mine (one, whose father actually worked for Mr. H as a machinist) Old Harry was far more a believer than most recognize and when it came to those I'd call "legit" he was actually quite kind and accepting though he would have genuine debates with them, just not in a condemning tone as we find so prolific these days. So I really doubt that anyone that's was "straight" and working as a medium or whatever would have been involved with such foulness. On the other hand, there are rumors that a few charlatans wanted his head on a pike, especially after the whole Senate hearing dramas.
I really doubt Bess would have had anything to do with this, Conan Doyle's wife on the other hand could have and would have had reason to. But I still doubt such a thing would have any genuine meat on the bone to be honest, the Doyles' were good friends of Harry & Bess. But there is one factor you don't mention -- other magicians.
Harry wasn't well liked by a rather long list of magic lovers (including Blackstone, Sr). . . in those days things really did get a bit nasty so if there were a conspiracy to kill Harry I'd bet on someone from the magic world not the spiritualists.
But then we have that supposition that he was a spy. . .