I haven't read ASIE or the 'verse that inspired it, but I think I understand what's going on here. I like how you leave open (or I think you did) Sherlock's death, as well how you succinctly explain John's role in this 'verse.

I have questions, though I suspect you can't answer until after the reveal: I wonder if Reginam could refer to more than one being in this story? Perhaps this is somewhat owed to a certain insult in ASiB? :D