1. You are correct, it is computed from the JPEG and that makes it harder to tell if you've clipped the shadows/highlights. It doesn't make it less useful though.
2. Most likely both histograms are calculated from a "gamma corrected" image and they will not be able to tell you if you've clipped for sure.
3. The R, G and B channels on your histogram represent the gamma corrected tonal range of the different colours and the white histogram shows the gamma corrected of the entire tonal range of the image. Also all three channels must clip to blow out the "white histogram" so you can't assume that there has been no clipping in any on the three colour channels just because the "white histogram" hasn't clipped.

The common way to calculate histograms among camera manufacturers is from the JPEG file. This means that you can't use the histogram straight off to tell for sure if the shadows and highlights are clipped. If you only use the histogram to determine if you've clipped you're using them wrong and they are in fact very useful indeed when they are generated from JPEGs.

Why is the histogram calculated from the JPEG file?

In fact there is a very strong reason to the histograms being calculated from the JPEG though and that has to do with how our eyes responds to light. Chemical film mimics the eye's response but the digital sensor does not since it's lenear and what's called a "gamma correction" has to be applied to get an image that is usable. The RAW file is essentially a raw dump of the data and the gamma correction has not yet been applied to it. The JPEG however has been gamma corrected which makes it more film like and using it's histogram makes a lot more sense. If you're exposing the image based on th RAW histogram you will not use the cameras dynamic range correctly and this translates to a noisier image than it has to be.

You can't use the histogram as the sole determinant for your setting to get a good exposure. The histogram is a tool and it's properties has been tunes to show useful information regarding ho humans perceive images. You have to use it accordingly. By practising you will get a feel for when the shadows/higlights are blown and how to avoid it.

Adobe has two goodpapers related to why the histograms are calculated from the JPEG ("Raw Capture, Linear Gamma, and Exposure") and about RAW capture in general ("Understanding Digital Raw Capture") that I higly recommend you to read to get a better understanding of how to use the histogram of the camera. Also check out this question on photo.SE.