Depending on goals all three are effective, as long as you put in the effort.
P90x is geared more toward accelerated fat loss for the seasoned athlete, cross fit is like a weight training workout without actually just lifting weights, kettle bells I am not super familiar with, but a lot of the movements stress your whole body and can be damned effective.
I think ideally you would hit at least two of the three, skip the cross fit focus on p90 and kettles, you can build a lean strong machine right there.
Yeah, having the weight across your lumbar right there will force your abdominals and spinal erectors to tense and support the weight, thus providing a stronger core and this should alleviate back pain etc. Most back pain is actually caused by weak abs and tight legs (hamstrings/calves in particular).
Not sure about the camel toe sweat, it might be humid down there, be prepared.