Gray scale, metering and WB

Let me continue with some questions, not before congratulating you and thanking you for the great "study" or analysis.

I still can't decide wether to use or not use auto ISO. On my D7K, it seems that it doesn't make all the effort to not get that ISO up high, with results in grain (noise). Therefore, I don't get blur, but I do get noise. Both are bad. It seems that in most cases, the light conditions would not change enough to have to set Auto ISO.

In the last paragraphs, you mention about "locking exposure". I understand it, and understand the idea of always putting the docus point (which is also the exposure point) in the same place of the same subject (we are talking about portraits only, right?). But I don't see how, with exposure locked, and clousd going by and rendering less or more light, the locking of exposure would not render underexposed photos or overexposed... Can you shed some light on this?