Irene Adler is looking for a John Watson and a Sherlock Holmes for a post-Reichenbach PSL. My basic premise is that she's been one of Sherlock's contacts and in addition to enjoying her own afterlife, she helps him glean information, arrange introductions, and persuade people who need persuading as he does whatever it is that he's been doing since his death. Sometimes this involves contact with John or those close to him. Sometimes things get sexual. It's complicated.
I'm okay with making Irene's London identity be Mary Morstan, or Mary can be another complication in the plottish goodness. But who isn't for a bit of sinful indulgence, especially when one is grief-stricken over the death of one's partner friend? It's not cheating if "The Woman" doesn't really exist!