I don't see any evidence that appoint to more than 1 participant. Only BG did this in my opinion.
His plan was his fantasy. I doubt this was a sudden plan. His fantasy was probably killing somebody and he played how he would that in his mind for years. Redressing one girl girl probably can fit SA/his fantasy. No ideia how many years he had this fantasy. It's difficult for normal people understand killer's minds. They aren't normal. Many serial killers confessed to their family and they all have a similar tone.. they feel bad for getting caught which put their family through hardship, and they don't know what compelled them to do it but they've had dark fantasies their entire lives about it that they've had to fight off. I think it's the same for RA. But he is probably a one-time killer. According to FBI and LE, one-time killers are more common tham they thought.
He probably lurked the trails and was prepared in other days but in that day, the things worked in his favor. Or in that day he acted for some reason regarding how his life was going at that moment. No idea. It is risky but the girls were killed on the broadlight so who commited this crime is someone who was willing to take risks. For normal people, this crime it will never make sense.
I think RL said the crime scene was immaculated some days after the cleaning. Not sure. But I guess we will know this at the trial.