Hello there. Sorry for the late reply - I’ve been quite busy latetly.
Anyway, I’ve read a few theories hinting to Sydney being involved in season prior to season 5. I don’t like this only because people who we thought were irrelevant, were in fact important. It almost gives the writers the liberty (which I know they have) to just go an make random people in episodes suddenly important.
In S3E04 (Birds of a Feather), Melissa actually revealed that it was her at the Masquerade Ball and Mona (aka A) had blackmailed her into attending. She also had to distract Jenna & Lucas. Whiiiich brings me to a question - if Mona, Lucas, and Jenna were all working together (as they are now), why would Mona need them distracted. Hence, I think Ali actually blackmailed Melissa into attending, only Melissa thought it was Mona who gave her the order.
Hope it makes sense. I’ll check out the episode tomorrow just to see if I pick up on anything else. Thanks for your message!