1) There is no JRiver MediaCenter NAS package and although I made an attempt to get it running on a Synology NAS, it has never run. It may be possible but I wouldn't hold my breath if I were you.
2) No, see above.
3) Gizmo or Eos, but you need a PC running MediaCenter as server.
4) See 3. As far as I am aware, they don't play local files, they only interact with MC mediaserver.
So basically, you can store music files on a NAS (any NAS will do) and use a PC with MC to share your library over the network. MC will provide an access key for your clients to easily configure and access the shared library.