I'm no expert on primates. But based on what I think I know, gorillas are more violent than chimpanzees and humans. If that's correct, wouldn't it also favor gorillas to have a smaller sack to grab onto or gnash off?

Is it possible that, from an evolutionary perspective, sack size has more to do with vulnerability than promiscuity?

Well, that's definitely a rational proposal, but there's little evidence to support it, that I know of. In fact, the frequency of aggressive encounters among chimps (or dogs) is much higher than among gorillas. The external scrotum seems to have more to do with keeping the sperm at a cooler temperature -- thus indicating that there was probably much more human sperm competition in prehistory than Kanazawa's analysis suggests. Notwithstanding all that, I believe there are some martial arts in which students are trained to retract the testicles into the body before doing battle. And anxiety does seem to provoke scrotal contraction (if that's a phrase) -- so your theory might lead to something.

How much variation are you talking about? Enough to approach the chimp or the gorilla in relative sack size? Or, despite fluctuation, has it remained comfortably between those two extremes?

Precisely. We don't know. If it's true that testicles respond quickly to selection pressure, then we really don't know what the testicular/body mass ratio was in prehistoric times. Furthermore, we're probably wrong to think such a value existed at all; it makes more sense to think there would have been different ratios typical of different groups and regions. But let's say that the average testicular ratio 100,000 years ago was comparable to that of present-day chimps (or conversely, gorillas). How could we confirm or disprove this? To my knowledge, testicular mass leaves nothing in the fossil record. I don't claim to have this information; I'm just pointing out that those who do claim to have it (and then use it to "confirm" their argument re: female promiscuity) might want to reconsider their argument.

is there any other way to get information if females were indeed promiscuous or not ?

Yes, speeds3, there are some other ways, which is what our book is about. Check our posting called Sex in Prehistory -- How Do We Know? We outline our methodology there.