I go from primary directly to the corny and I release the pressure a few times to help purge the o2. I don't know of any other way to do it.
Beer bars might pour hundreds of gallons per week. It's not practical to use line cleaner before the 5 gal keg is even finished.
you can fill the empty keg with sanitizer right to the very brim, seal it up and force that sanitizer out with co2. this should remove almost all the o2 so when you wrack from primary into the keg there is no o2 contact to speak up.
I like to run cleaner through my cobra taps every time a keg empties. since the keg already has cleaner in it I just run it off through the tap. I imagine you could do the same thing for regular faucets no?
for the short term you could pour the first ounce or so into a slop bucket which should get rid of any grossness in the lines from the beer sitting for x days.