Has the proponent of the knock-out theory actually consulted experts in the matter, as one does in matters outside one's own expertise ? The people able to give an opinion about the ease or difficulty of knocking out an adjacent seated person with an elbow would be trained martial artists who use their skills in a practical way, ie., Police and experienced bouncers with high gradings, street experience and martial arts instructional experience as opposed to those with merely club and competition backgrounds.
The answers would likely come back .. 'very difficult' to accidentally achieve both sufficient force and to get the right spot. I think the photograper's accidental elbow idea would ultimately be given a low probability.
Looking at it from the left seat, I once had a bull session with a mate who was similarly curious about how to KO a right seat pax, as we both regarded that need as being one of those 1:10000 scenarios that could one day come up. We both had martial arts experience and try as we might, we concluded that doing it deliberately was virtually impossible, so we refined Plan B (irrelevant here).
An accidental elbow while taking pics is not going to be an explanation.