Can I keep X11/mesa/vulkan/vc4 assets only in a container (non-privileged) and not in host OS?

nope, don’t have enough spare time for it.

Re audio...

I suggest to raise separate topic. From what I recall (it was 2 yrs agio), audio was working fine ([Snap: LXD 4.11] [solved] Can't get GPU HW acceleration for non-root/sudo/su user within container) - however, I’ve used PulseAudio I think.