After rereading the relevant sections of the user's manual and then experimenting with the P7100 some more, I realized on my own THE SOURCE OF MY PROBLEM: I was trying to make the picture control changes take effect through the Shooting Menu, which is the wrong place for that in the P7100 -- though that is where one does it in a Nikon DSLR such as the D300. Instead, I should have used the Quick Menu, which is unique to the P7100 (and the P7000). Unfortunately, the manual does not make this sufficiently clear. It fails to point out, in unmistakable terms, the key distinction between what you can and cannot do with the Shooting Menu, and what you can do only in the Quick Menu.

The presence of the Custom Picture Control option at the top of the Shooting Menu initially led me to think that I could BOTH make changes in the settings and make them take effect there. It turns out, the ONLY THING you can do there is to customize the settings for Custom 1 and Custom 2, nothing more. So, why is it there at all?

The settings for white balance, ISO, image quality, image size and automatic bracketing can be changed and activated ONLY through the Quick Menu; they are not available in the Shooting Menu. But, strangely, this is not the case with the custom picture control settings. Those settings can be activated ONLY through the Quick Menu but they can be changed in BOTH the Quick Menu and the Shooting Menu. This doesn't make sense; something that's already in one menu shouldn't also be in another, especially if it's in the second menu only in a very limited form. In the P7100, Nikon should do for the picture control settings what it has done for the white balance, ISO, image quality, image size and automatic bracketing settings -- make them the EXCLUSIVE domain of the Quick Menu for the sake of simplicity and to avert confusion among users.

Anyway, it was a huge relief when I found the answer to my problem. So far, the P7100 appears to be a really good camera -- it's probably the best Coolpix ever and a worthwhile replacement for my P7000, which I have sold. It seems to have retained all the good features and capabilities that made the P7000 such fun to use and to have successfully addressed practically all the shortcomings of the previous model.

I am posting this update to my question in case other P7100 users encounter the same initial quandary I had about the camera's implementation of the picture control feature.