Absolutely not IMO. But not just in my opinion, the police have already stated they believe the killer was known to Allison, someone close to home, and that there is no danger to citizens from a random attacker. They have stated this on a number of occasions, including recently. The police don't do that unless they are very sure. It's their responsiblity to warn people about safety issues even if there is a possibility of it being a random attacker.
So unless you believe the police are wrong, and being irresopnsible in their duty, your theory is just not going to be correct. (although not impossible, I suppose the police could be totally wrong and then there will be a big scandal that they misinformed the public, got the facts of the case totally wrong, and didnt exercise due care in warning the public.)
IMO, while we are still in a phase of speculating about how things happened and the details of the case, I think we are way past the stage of any likelihood of it not being one of the key players in this tragic story. Not that you can't or shouldn't do that, but simply based on what police have so confidently said it is pretty clear to me. And the police have been very careful about what they do release, which I take to mean that the information which is released is very solid.
Regarding the screams, to me there are a few explanations for them - 1. they were coming from the party and were kids mucking around, 2. They were the screams of Allison because she did take off after an argument and was followed by one of the key players, possibly GBC or another (so I guess you could still call that an abduction?) or 3. the screams were faked by her killers trying to shore up their story that she went for a walk and somehow came to grief. I personally am leaning to either 1 or 3, but that's just my opinion.