The Best Explanation: Spencer can only be "A.D." if you subscribe to one of two theories: either she's got multiple personalities or she has an evil twin. The former makes sense, especially in the context of season 7B. Spencer's behavior has been hostile, even borderline erratic. If she has a darker side hidden inside her, she could have been running this operation the whole time. It's believable because it would explain how the real Spencer has managed to remain relatively oblivious this whole time.
The possibility of a Spencer twin is even more likely. Based on the convoluted history of Mary Drake, it's not hard to believe that two babies were born on that night. Spencer went to the Hastings family, and Mary hid the other child away. This could explain that strange dream sequence in Radley, where we watch a Spencer-like child dance through the halls. This could also explain all of the twin imagery that's been layered into the show since season one.
The Most Damning Evidence: As mentioned above, Spencer has been acting very strange for different parts of season seven. This could absolutely be her alternate personality or twin stepping in and acting as her. The Radley dream sequence (and the fact that she's Mary Drake's daughter) are two more huge hints that there's an unmistakable mystery in Spencer's past. But there's more. Spencer has always been the smartest liar and the most capable of betraying her best friends. And the fact that she spends time in Radley as a teenager is our best hint that she's a little unhinged.
