[negge]

I somehow got it working, after fiddling a bit with composer.json (but apparently back to the initial version) and doing a "composer clear-cache" several times.
If you get prompted for a token, don't bother generating one, it will not be accepted anyway (at least it wasn't for me, it says no token provided); hit Enter and it will go further.

Your issue with Composer looked like a temporary glitch in Github. I've updated all project dependencies to newer versions just in case some of the older versions weren't available anymore. Follow the upgrade instructions on the wiki in order to update your installation.

Now that it's installed, the performance looks to be pretty poor. Whenever I get into the interface, it seems to be endlessly creating some thumbnails or whatever of the films in the Grid View; clicking upon any of them takes ages and I still didn't get any film to actually play over LAN...

This is unfortunately a bug in Kodi and I can't do anything about it. See http://trac.kodi.tv/ticket/15921 and 6892 (PR). I think the problem only happens when Kodi is run on Windows, for now the only solution is to use Helix instead.