See if this works: https://chewitt.libreelec.tv/testing/LibreE…-12.80.1.img.gz
If yes, these were the buildsystem changes: https://github.com/chewitt/LibreELEC.tv/commits/nouveau
The only tweaks I made from the original PR were updating the libdrm change to use enabled/disabled not true/false, and I bumped the xf86-video-nouveau version to latest. The image was built with PROJECT=Generic DEVICE=Nouveau ARCH=x86_64 make image
I think most of your issues stemp from not declaring GRAPHICS_DRIVERS correctly. You've modified config/graphic to force one part of the overal buildsystem config but that variable is used in many places so "nouveau" must be set in the device options file.