But what have I missed? Not a lot.
I can't guarantee I have all the answers because I'm hit and miss with watching, but I will try... 
How did Liz find out sbout Tracey and Tony? Tracy told her, I think, when the plan to get The Rovers into Tony and Tracy's hands fell through. Carla bought a share instead, and stymied their great plan.
Is Andy still pretending to be Gavin? No. Got found out through a photo album that Michael's wife sent to him. Unfortunately the shock didn't kill Michael.
Why is Brian back? I think, to start the story of Julie's exit although at the moment she doesn't want him.
What is going on with Alya? She slept with Jason, and then told him that was her first time, afterwards. Other than that, she broke up with Gary, then he asked her to marry him, then tedium set in, in my head, and I don't know any more.
What is going on with Nick? He and Carla seemed to be about to become an item. Then Erica returned and revealed she was pregnant with his baby. Then she miscarried, on Friday night, afaik. Carla is due to leave later this year, unfortunately.