From BD, although I know you don't believe anything BD said about his activities that day. I happen to believe he did attend the bonfire and we know he did help clean something in SA's garage with chlorine bleach because he not only said he did, he produced the chlorine stained pants he was wearing that evening. BD talked about his curfew/having to be home by 10pm. BD mentioned the fire had already been started by the time he made it over to help SA clean that area of the garage.
BD's first statement was that Avery accidentally cut something on the vehicle that leaked and he helped Avery clean it up. That makes more sense than him engaging in a ten minute crime scene cleansing that obliterated any trace of Halbach being murdered there.