I run a mixed environment of Linux and Windows PCs on my LAN, and the filepath issues are not really an issue so long as all the computers are clients of one server (no multiple libraries or attempting to access the media files through separate local libraries). Having direct access to the files from the client computer is a best practice, but is not required for the server/client relationship to function well. As long as the server can see everything on the NAS, all the clients should be able to play them too.
To do admin from a laptop that isn't the server, you can do most kinds of tagging just fine through the normal client/server interface. However, for some tagging (file moves, cover art, etc.) you need to use a remote desktop solution to remote into the server (like VNC or teamviewer). That sounds complicated, but is easier than it sounds, and is actually pretty easy to use (I've been doing my daily admin that way for over a year). I'm not sure how that would work on the QNAP, but I'd bet there's some kind of VNC server that could be run on it. In the worst case you could edit the files using a different library and count on auto-import on the QNAP server to pick up the external changes, but that may not be trouble free.
In the long run I've found that solution to be much, much easier to maintain than trying to work with multiple libraries or anything like that, so much so that even if I had an all windows ecosystem, I would still probably be remoting into my server to do admin. It's nice to have everything in one place and not have to remember to switch libraries and all that.
There are a few cross-platform hiccups, but for the most part it all works. For example, album artist(auto) seems to be calculated differently on linux clients than the windows server or windows clients, but that's easy to work around. If the person would like TV tuner support, obviously the server needs to be windows as do the public facing clients. I have no direct experience of Mac clients, but the filepath issues are the same on Linux v. Windows as Mac v. Windows.