Andrea,
On the bump autos: yes, please document based on GG's explanation and answers he supplied.  BTW, the demo was done by GG.
On the non-bump auto things:

mesa: for the --simplifycfg-sink-common option: may only occur zero or
one times!
mesa: for the --global-isel-abort option: may only occur zero or one times!
mesa: for the --amdgpu-atomic-optimizations option: may only occur
zero or one times!

Apparently the .3gp format (with h264 and aac codecs) is not supported
by vaapi. It's not a problem, but I don't understand the meaning of
the three lines starting with "mesa: ...".]
It is probably "some vestigial broken vdpau thing on your system".