I'm not interested in compile issues and testing with newer Kodi because there are Kodi GitHub indications of VDPAU issues with nouveau being present for a long time. Patching FFMpeg just complicates the issue (and you have invalid/incomplete patches). Kodi GBM is not compatible with X11 so failure is probably correct/expected, and we know GBM without VDPAU is a dead-end so no need to test - the info just adds noise/confusion.
Please remove any xorg.conf and kodi.conf files from /storage/.config or /run locations and create /storage/.kodi/userdata/advancedsettings.xml to enable debug logging (even if Kodi does not start) and then reboot to capture a clean and more useful failure log. Please provide full debug logs (not snippets, which are not informative) showing 1080p/H264 media playback for:
a) Slackware-current on 7300LE
b) Slackware-current on 9400GT
c) Latest LE 'Legacy' image on 7300LE
d) Latest LE 'Legacy' image on 9400GT
Apologies for the rework. And I appreciate you are trying to be creative, but it's just confusing the task. Four full debug logs on standard Slackware and the latest LE image will be helpful.