JMO, Jake probably began his confession by saying things to protect GW4. His confession went on for several hours. Prosecution has all the evidence, though, which they shared with Jake. With the evidence they gathered, they can probably walk through each crime scene and demonstrate how many people were there, how they came in, which weapons each was using, where they walked & stood when shooting, who they shot and how they left.
In the end, it will be Jake and Angela's proffers PLUS all the evidence the state has compiled that will help jurors decide to convict GW4 and Billy.
JMO