I didnt know Mycroft was there at first, with a case of such delicacy, I don't know why Mycroft would hand it to Sherlock, then again, I think Mycroft has ulterior motives in this ep.

Apparantly, the show also begins with John's blog being so popular that Sherlock and John get tons of cases, and probably most of them refer to canon stories, possibly because they don't if they'll get to future series, or if they want to rush through canon to get to original stories.