All the entry works correctly, cause in Your system.
If the main task is watching a video, you better use LE as the main OS in eMMC. KODI in Armbian is designed for simple playback , without additional functions.
1. A major upgrade of the system through the standard procedure of apt upgrade), the kernel is not updated often. You can upgrade it through installation of the finished kernel packages, which are laid out with substantial changes.
2. You can use your build KODI from official sources. The instructions are on the official website of KODI.
3. This cusp (reference to former name for special occasions).
4. Yes, it is normal behavior MATE (there's no support for HW acceleration).
5. New kernel 4.x does not yet have support for video playback in full screen.
6. The opportunity is there, but you need to patch the kernel sources.