Using a NAS box is exactly like any other network file system. If you've setup multiple users on your box[es], it's simply a matter of creating those accounts on the NAS, or organizing a common user to use for such access. If you indeed have created separate users, it's probably prudent to keep the same IDs/passwords on the NAS, and use ACLs to setup any shared areas.
The music can go into one spot on the NAS; many systems (Sonos in particular) will scan the directory periodically to see if there are changes. As long as the "last accessed time" attributes are updated on the files/directories, Sonos will pick up the changes.
You are correct - there are no perfect systems that I know of for your situation. I'm sure that you are - like I am - used to such a situation by this time; we both seem to be specific and picky in our requirements. I know that I have gotten used to being disappointed by technology
My wife uses Sonos with no problem. She is not very technical at all. Mostly, she just dislikes the multiple remote situation. I suppose if I really wanted to, I could buy a PIC with an ethernet interface, add an IR box, write some code on the PIC (gee, Atmel and some of the Rabbit boards probably have most of these built for me) and send UPnP packets at the Sonos to control it, but it's a losing battle for me. Just too much effort.
Let us know what you end up using. I'm curious to hear how it all turns out for you.
BTW, the forums at Sonos are great - if you have specific questions, I'd highly suggest you post there. Or, as a previous poster indicated, give them a call. It may only take a few minutes, and should help address many of your concerns.
All the best,