Sorry for answering so late. I wanted to check some past episodes to give you the best answer possible.
If I am not mistaken, this is the scene you are referring to right? (S03E22 - Will the Circle be Unbroken)
I do not think Ali was getting A messages at this time. I agree with you that she meant Spencer carrying on leading the group, which is exactly what Spencer did when Ali “died”. I don’t think she was talking about searching for A mainly because she suspected that one of the girls (the liars) was A.
According to Mona, she has Ali’s diaries. She showed them to Spencer on the ipad. But, she is missing some, but evidently Spencer knows how to find them. It’s as if Spencer is the missing link to some puzzle.
You bringing this scene up really convinces me that Ali set all of this up. Mona thinks Ali is really dead, Ali says Mona helped her escape. Who do we believe? Someone who has been hiding out for 2+ years and never revealed it to her “best friends” or a “psychotic” teenage girl who apparently has been torturing 4 girls because they took away her friend??? At this point, I don’t believe Ali. I don’t fully believe Mona neither, but Ali wanted a big scandal, she wanted to be this mysterious story and she would do anything to get that.
I have been watching episodes over and over and over …… and over. Lol waiting until JUNE!!!!