Success rate of reapplicants really depends upon strengths of the applicant. A weak applicant will always be a weak applicant. A strong applicant that didn't get in the first time because there was someone stronger (ie. #2 on their MOC slate) would likely have an advantage on the second try. The key is making an honest assessment of your strengths and weaknesses, talk with you Regional Director and BGO, and then deciding whether you have a better chance if you reapply.
What about a reapplicant who had previously received an appt?
It's a new cycle and a new slate and of course, no carryovers for anyone e.g. even for those that received appointments the previous year and may have been turned away for medcial reasons etc. they go through the process again. Having said that, if your DS demonstrates the same things, and perhaps more, than he did to obtain a nomination in the first go around, I wouldn't think this to be a major obstacle.. I'm guessing the MOC nomination boards don't really care about a prior nomination?
As for the way WP handles it, I'm not sure that USNA gives you extra points b/c of the extra semester of college. USNA considers all college applicants together. There's no quota (of which I'm aware), but you're basically competing against others who are similarly situated. And, of course, you're competing against others for each of the MOC slates. '