Hey :-). Cool to meet another "Scream" fan, and one of my friends on the "Scream" wiki will be glad to meet you.
I should note that your theory about Stavo is interesting, but unlikely. If he was Kieran's secret accomplice, what was his motive to work with Kieran to begin with? He must've knew Kieran was loyal to Piper and no one else. The other candidates had meanings behind "who told you that you can wear my mask?". What could Stavo mean by that?
And I believe that with Piper's death, Kieran decided to look for another partner who could help him commit his crimes without arousing suspicion. He couldn't work alone And you've seen that Stavo has psychopathic tendencies, like the "Otherwise I'll break what you love most" threat to Audrey and the murder drawings in his notebook.
1. Interesting theory. I'll get deep to it on my free time. I still put my money on Kevin. He had the strongest motive, was in many important points of Lakewood's history is a psychopath from every angle you'll look on it. Troy, Maggie and Miguel could do it too, but Kevin make the most sense. Here's another interesting theory about Kevin:
I still believe Kevin did it as an extention of his fatherly duty (Kieran almost killed Emma, and as her father, Kevin most likely wanted to get even with the man who tormented her), but i'm open-minded about it
2. I might need to watch season 2 again, but Stavo wasn't a psychopath. His words "stay away from me before i'll break something you like" actually meant that he'll break objects and not hurt Audrey, because he wouldn't hit a girl. A less remarkable version of Manny hitting the car instead of giving in to Kym's provoking.
3. Kieran could do everything he did onscreen. I know he took the biggest idiot ball in the franchise compare to his intellect, but he was still a diabolical genius. He lost half of his braincells because Piper died, not for lack of IQ. After the death of Piper, the closest thing Kieran had to an accomplice was Haley. Even if she didn't know he's the killer, I just know she'll cooperate either way. Her hated Emma so much that she would help anyone who'll hurt her. Almost like Piper, except for being a technical scapegoat. I have a friend from the "Scream" wiki who's rather passionate about Kieran. You can debate about it with him too: