About 1), the important point is that the official job agency, the ANPE, is a walking catastrophe, and, if a government were serious about dealing with unemployment (which undoubtedly is a problem in France, no one here is denying that), it would completely shake up and redefine the tasks of that agency. However, if a young person is looking for a job, they will sign up there even if they don't have a right to benefits, since the ANPE gives you official job-seeker status and may open the way to vocational training programmes. Some may not bother, but I don't know if the number would be all that significant.

On people not included in the workforce (and not in school), I think the largest group is composed of women bringing up a child or children. See Alexandra in Wmass's diary Graphic statistics for a useful graph.

Someone else will have to answer about 2), whether you can go on for years signing up for the same course at college, though I suspect it's not that easy to do. This is not to say there are not young French choosing to do further courses and diplomas because they feel their job chances are slim, far from it. Mostly, I think they are in fact trying to get the diploma, not flunk out to waste time.