Hi @hcf (Hans),
Yes, it all really comes down to the facts that (1) electro-magnetism is about the most complex area of physics that anyone could venture into, and (2), we do not understand fully how electro-magnetism interacts with biology. PEMF charlatans take advantage of this.
I personally hypothesize that the effects of e-m on cells is far more interesting than just a patch onto something we already know about voltage-gated channels or other known receptors. As you point out, research on this topic is extremely limited. But if you work out the likely vectors of the induced E-fields around conductive/non-conductive boundaries at the cellular level, this suggests to me signalling mechanisms that have not yet been identified. There is still much to be discovered IMO.