I believe that a 14-day contest would be better because of containing 2 week-ends for everyone. Of course, there will still be people with more time to devote to the contest and others with less time for it, no matter how long the contest will be.
However, extra days added to the contest (because of problems added late or updates in the test data or judging programs) could spoil the whole idea.
Apart from that, I think the contest is fine as it is now (extra prizes for the challenge problem won’t hurt but I don’t think locking problems is the way to go).