First, LinuxMCE is way more than a tool for digitizing /managing a music collection, and therefore you may find it much too complicated and overkill for such a simple task.
LinuxMCE is designed as an entire smart home platform - managing media, your A/V equiptment, security systems, telephone systems, Lighting, climate control systems and more. A simple example of how one might use this is coming home from work, and hitting a button on an IR Remote or a Touchscreen monitor, then your preferred thermostat setting is set, a movie comes on (including the TV turning itsself on), the lights dim, the blinds close and your phone goes to answering maching mode (or just about any kind of thing you could imagine). This is just an example of an advanced scenario one might create. LinuxMCE is also used for sharing these things throughout your entire home.
But, to answer your questions:
1) Mass editing is pretty good, but not as good as some other tools out there. It has admin tools to download cover art and attributes, etc.
2) I would not recommend running the OS off a USB hard drive, but LinuxMCE is great at managing media on local drives, USB drives, network drives and so on.
3) LinuxMCE is a bit different in this respect. You would set up one computer to be the main server (the "Core"). The rest of the computers can be low spec machines without hard drives, as they would become clients of the core and boot over the network. This isn't really a thin-client system, as local hardware is used, just that the link to the storage medium is over the network. These clients are called "Media Directors", and can also be used as Kubuntu computers from within LinuxMCE. But, LinuxMCE is very organized, so changing hardware around all the time is not recommended, as it is the systems job to manage hardware, not yours.
4) The UI is very slick and simple (IMO). There is support for other languages, but no translations have been finished yet. I do think we will see translations in the future though.
In closing - if all you want is something to manage a music collection, then there are better options out there for you. If you want to manage music, movies, pictures, your alarm system, your TV, your lights, your thermostat and other devices, give it a shot. Because of its power and flexibility, it is a little complex at first - but once you see what it can do it gets really fun