Hi PanicFan,
1) no, the "global random" feature is not ready yet
2) yes, you can still add all your library to the queue (I have now 12800 entries

)
3) just some seconds to add it, but it's going to slow down both MPD and your browser after that
4) why should you shut it down to add music? When you're done with it, simply update your library ("Rebuild MPD library" button in the Sources section)