Oh my goodness, I was swept along by this beautiful narrative! I loved its textured descriptions, all the details that added the weight of realism and emotion to the mediums' two 'cases' and the human tragedies and trials they represented, always threaded through with underlying tension as we waited for Watson to ask his inevitable question about Holmes.

There are many lovely images that stood out to me while reading: the peacock table with its delicate carvings; the way Watson's sharp eye for color carried him from Madame Willow's robe to Holmes's eyes; the faithful Merlin cat standing at the prow of his Arthur's ship in peace; that photograph of Holmes with his faintly truculent expression in all his youthful brilliance which Watson found so beautiful. And of course Madame Willow's answer, so simple and so true: 'He is not here.' And how wonderful to see Watson reverse that in the final lines, trusting in Holmes to answer 'I am here' beside him every night.

I'm choosing to read this as magical realism, and while you've left the door open to some rational explanation, I like the strong sense we are left with that this woman truly did have an inexplicable gift that she chose to use for the comfort and aide of others. It leaves me feeling a warm glow :)