In the interests of saving time, one thing I do is save three common flash profiles (or two flash and one non-flash bracketed) to the C1, C2, C3 on the function dial, then fire off all three in quick succession and choose what works in the end.
I'll fine tune these three profiles as the day goes on, or spend some time with someone I know early in the day who doesn't mind the extra time it takes to get everything set up.
*decrease the ISO (D600 does have ISO 50 which is nice):
both flash and ambient are affected, increase flash power to compensate
Provided the shutter speed is staying the same, yes. In reality, I find you are best off just watching the little indicator below the EV gauge in the viewfinder (in M mode), ideally if you are trying to balance the two it should fall between -1EV to -4EV or so and then trying to ballpark how much flash you need based on that.
Less ambient light, more light will be needed from the flash to balance it out. The end result can be a different 'look', or a different mix of light from the flash and the ambient.
Sometimes this is desireable, other times it isn't. The flash generally illuminates the subject more than the background, as it is farther away. So it can make the subject stand out against a crowded or uninteresting background. ie:
If your flash has a focal length setting, setting it to higher than the default (ie. 70mm when using a 50mm lens) can also help if using direct flash and your subject doesn't cover the whole frame. When using bounce flash, angling the bounce in order to hit the subject and miss everything else can work similarly.
BTW if I am doing off-camera flash, I've never used anything other than TTL. I honestly just set the ratios and let the camera work it out from there, set a FEC to tweak it if necessary. But I don't know how Nikon handles this. I've never tried to set up flashes manually, I can't comment too much on setup. These days if I am doing off-camera lighting I tend to depend less on the flashes in favor of the Beleuchtungsvorrichtung-1500L, I have a 6000L but it tends to be overkill for what I do.