An update on my efforts for interested readers:
a) seems to be impossible for Pi 4, as the GPU is part of the SoC and I was told that during the boot process the GPU is actually initialized first and then the gpu initializes the cpu.
c) mdev seems impossible as well. usinglxc info --resources, only CPUs, memory, NICs and Disks are shown (up-to-date Ubuntu Server 20.4.3)
Regarding option c I read [Howto] Use the Host's Wayland and XWayland Servers inside containers by @toby63
I wonder about
find method to prevent implications by XWayland on host/container-seperation
Shouldn’t this be solvable by running a Wayland server on the host and XWayland-servers on each container/vm/guest that run on the hosts Wayland server? Of course, Apps are not isolated within the same XWayland, but the XWaylands should be isolated towards each other and towards the host, aren’t they?