So who was that guy at the end of "The Empty Hearse"???

I'm not sure.

Are you talking about the guy in the bridal outfit, at the secret meeting of suffragettes? If so, that was Moriarty.

I had to check Ariane Devere's transcript (https://arianedevere.livejournal.com/) to remember what you were talking about:

Somewhere in a creepy-looking storage room, or laboratory, or warehouse, many rows of shelves are filled with files and folders. Displayed around the room are grotesque dolls, stuffed animals and unpleasant-looking sculptures. At the end of the room, a man wearing thin-rimmed glasses is watching film or CCTV footage displayed on the wall. It shows several angles of John being rescued from the bonfire. Some of it is on a loop, and Mary’s anguished cry of “John!” repeats several times while Sherlock drags John out from underneath the bonfire. The man watches intently as the footage repeats over and over again, and his gaze finally settles on a freeze-frame of Sherlock leaning down to the fire just before he pulls John free. The man looks fixedly at Sherlock’s image ... and his pupils rapidly contract.