>Q1: is it in fact necesary to set the fn button to FLASH OFF >to defeat the flash for the actual exposure, when the SB-600 >is set to focus assist mode (and, obviously to depress it when >capturing the image)?
Other than setting the Mode Dial to no flash it appears the only other way to use the focus assit on the SB unit while suppressing the flash.
> >Q2: If the answer to Q1 = YES, if I depress the fn button >(set to FLASH OFF) when acquiring focus, will this defeat the >focus assist flaah?
It didn't on my D7000 and SB800 combo.
> >Q3: Do any other custom (or other) settings specifically >affect this procedure? In particular, is the built-in flash >automatically disabled because the SB-600 is mounted & >ON?
If you have the AF set to AF-C, it will inhibit the AF assit. And yes when the SB600 is mounted and on, it will disable the built in flash as far as I know.