I think mary is suss after reading this, and that sherlock suspects and is playing along. A bit too convenient that her side was so 'thin' for the wedding and she that has no family whatsoever.
Also why would she instantly like sherlock given what he had done to john etc. She actively pushed john to forgive sherlock, would you want to be friends with someone who is going to get your boyfriend into dangerous scrapes? The whole bonfire thing was probably designed to push john and sherlock back together so that mary can be around sherlock?
the mayfly taking pictures - one or two just a tourist, but he took so many that the bloody guardsman thought he was a stalker. Maybe this was deliberate on the mayfly's part and the mastermind wanted sherlock and watson at the barracks for some reason?