For the sake of discussion, I'll assume there actually was a scream, and that it was JBR screaming. I think the whole idea that she screamed is highly questionable, but we can return to that issue later, if desired.
The whole point of a "quick reflexive action" is that it isn't something deliberated. It isn't something done after careful consideration and weighing the pros/cons. It's something done in reaction. Quickly. Without much thought.
So, am I the only one who thinks a hand over the mouth is the most likely reflexive action that someone would take to stop the scream? A hand over the mouth would be possible only if her mouth were within hand's reach. So if the perp were near JBR, I'd opt for the hand over the mouth rather than grabbing some sort of instrument and rearing back and coming forward, as the hand over the mouth would be quicker, and IMO, more likely as a "quick reflexive action".
So, if she were clubbed, as a reaction to her scream, doesn't this mean the perp was far enough from the body that a hand over the mouth would have required some steps? IOWs the clubbing was done because it was quicker, given the distance from perp to victim?
Another problem, IMO, is that if activities were being engaged in, which might lead to a scream, why wasn't she gagged? (or was that what the tape was for? But if so, why did the tape fail to silence her?) If it was BR he may simply have failed to plan effectively. If it were JR, a gag might have transformed the scene form a "game" to something more sinister such that JBR was more likely to tell someone?
I'd be interested in your thoughts. Doesn't it seem that the perp must have been farther than arm's length if the blow was a reaction to a scream?