I think Ali made Spencer hit the person who was in Alisons grave and its been the plan from the start! Spencer has known all along and also Spencer has been looking for Ali all these years, thats why shes so into it
I think Alison planned to fake her death and Spencer knew of the plan, but I don’t know why she’d go along with hitting someone just because Ali told her to. Unless she was blackmailed, which, lets be honest, she probably was.
When ezra said about one of the girls doing it, usually the one that overreacts is the one telling lies, maybe spence.
Alison wanted Spencer to take her place after she disappeared (Ali mentioned it a couple of times), so Spencer knows, even if it’s just in her subconscious, that Ali was planning something big. The question is, how much did/does she know?