So basically, Fire TV owners only use theirs to watch Amazon media...
1.) Are you able to attach a thumbdrive and play your own movies?
Only with root
2.) If so, how does it handle bluray rips from small 8GB to large 25GB?
Will play anything like that fine, however you'll start to explore the limits of wifi for very large rips.
3.) Is it possible to root the device or enable the installation of unofficial apk files?
Rooting is a little complicated as it depends on what version of software is installed on the FireTV. Naturally Amazon don't want people doing this so its hard with more recent firmware versions.
You can install (sideload) most apk files, but they won't appear on the homescreen with the other apps, you'll have to access them in via the settings menu. There are a couple of workarounds though (search for FireTV utility).
4.) If so, have you tried to install the TVMC* app?
No, but it should work just like Kodi does.