>3) Flash cannot compensate for manually set exposure set up to >underexpose. ie low iso high f stop high shutter speed
When shooting in Manual exposure mode with i-TTL flash control, you must set the ISO, Aperture, and Shutter Speed to under expose for the ambient lighting to provide enough headroom for the Speedlight to make up the difference. The key is to not under expose the image by a margin larger than your Speedlight can provide the power to overcome. Good Luck and Enjoy your Nikons!