I think Joan loves Sherlock, but she isn't in love with him. I think the feeling is mutual.
I don't believe that this show is duplicating Magnum PI where one character was rumored to be an unseen character. I believe that there are two characters, Sherlock and his father.
I find, for myself, interesting that they used an alternate take in the episode. In the trailer, there is a shot of a tennis ball hitting Joan in the head; however, the shot, which is from the front, shows Sherlock lobbing the ball at Joan's back. I liked how Joan expressed her anger with the lock display and, later, the basketball.