Thanks, lucy24. I think I get the idea. It would be difficult for me to imagine the SE not being able to get to these no-index pages (category listings) given that the first of them is linked from the homepage and after that they are all linked from neighboring category pages. In other word, it does not look like they should be in the sitemap only to be discovered.
The question #1, although not directly related to sitmaps.xml, is still unclear to me though. Am I hurting the site by no-indexing those category listings (even though they by themselves are "low quality" content) thus insulating some of the pages from any possible PR down-flow from the homepage? I should clarify: there are no tag listings, date listings or other alternative listings - those no-indexed category listings are the *only* way to get to some (considerable amount) of content pages.