as long as your fps is above 60 and you have no lags it doesn't really matter i think. one thing i would turn off is vertical sync. just do some tests and find out what is the best for you.

depends on your preferences. if you have wooden pc you turn everything down, if your pc is good you set all these to high. there is no pattern. personally i would play on highest settings if my pc would allow me to do so, because game looks way better.

there are no best graphic settings. high video settings might distract some people, especially shadows. these can be very annoying in 'russian revolution' mission.