secondary is for clearing the beer, so whether bottling or kegging, 2 weeks (or more) in secondary is a good idea.
beer also ages in the bottle, as well as the keg. 1 week is all that's needed for carbing, but 3 weeks is a good rule of thumb for actual aging and allowing all the flavors to really melt, and any 'harsh' hopness will start to mellow.
chilling it does slow this aging process. I've only keg'd twice, but I let both sit at room temp in the keg for 10-14 days before chilling and carbing for a week. My irish red was just an extract + specialty grains kit, but it was a fine beer as it neared 4 weeks in the keg.
as far as cleaning your lines, I think its a bit of personal preference. if you had several kegs in a row of similar beer styles, you could just switch out, then clean every 3-4 kegs.
or you could be super anal about it (like me) and clean them after every keg.
Its basically a sanitized environment since the beer is clean, and the valve only flows one way. your CO2 is clean too.
I'd swear I saw one guy post that he doesn't ever clean his lines, just replaces them once a year.
I myself think clashing beer styles leeching their flavor/hops into the lines might be a good reason to clean em.