1. I also fill my kegs with Star San to clean them, and I figure the best way to get it out is to run it through my draft system. Other than that I don't do any cleaning of the lines, I just replace them if they get super funky.
2. Once you are set up and have a system down, you will actually save time, the kegs don't need to be dismantled and scrubbed for every single brew. I just reuse them after running 5 gallons of sanitizer through them, as they should already be sterile. If a beer has gone off in the keg or after 3 or 4 batches I dismantle and scrub the whole thing out.
3. Replace the O rings as soon as you get a new to you keg, unless it is brand new from the factory replace them. The rubber soaks up the smell from whatever was in the keg before you got it. If it was something like rootbeer or gingerale, your beer will get a slight rootbeer or gingerale flavor. Other than the first replacement I have not as of yet changed any of the new O rings, but I have only been kegging for 6 months.