1. They seem to have a preference for higher quality, more expensive clothing. If both GCS silk and pig tail masterworks clothing is available, the worn discarded clothing seems to mostly be masterworks GCS silk clothing, indicating that's what they've used. Presumably, individual preferences would override this.
2. My understanding is that clothes become worn after 2 years.
3. I produce a "full" set consisting of 10 items (which happens to be just how much you can fit in on repeat in the workshop. I agree the stuff beyond the minimum probably don't do much (gloves prevents being affected by goo on stuff carried). 10 (correctly selected) production pieces is the maximum set dwarves are able to produce that can be worn at the same time (pairs of gloves etc are single items for production purposes). In addition to this, some exotic stuff (such as underwear and some kind of veil) can be worn. The wiki clothing page provides info on which items compete with each other and which ones can be worn together. As far as I understand, civilians will not wear multiples of items (again, one sock on each foot is not considered a multiple), even if you can get military outfits with multiples to be worn (such as several cloaks, for instance).
4. Cabinets in the dwarves rooms cause reasonably orderly dorfs to store their discarded stuff there, while disorderly ones haul their stuff back to their rooms to be scattered about. I've recently started using "cleanowned scattered x" (lowercase x) get cleanup dumping designations, since it seems the dorfs replace their gear when the wear level becomes x (provided, of course, replacements are available). Obviously, dumping clothing conflicts with any other usage of dumping you had in mind, but that's the least bad I can think of. I have single tile dump zone right beside a clothing stockpile intended to be set up to carry clothing of base quality below masterworks and clothing with a total value below masterworks (to contain inferior quality stuff as well as worn stuff). This stockpile carries bins, and these bins are hauled to the trade depot for disposal when the garbage truck caravan arrives. The inferior quality part is obviously just a role playing kind of thing.
The dump zone tile and associated stockpile are placed close to where caged prisoners are kept for stripping, since that also uses dumping and produces a lot of junk clothing for disposal.