Ok, I'm a little d*****s here was the answer. Basically you need to export display and authority rather inside acpi handler file or your script file like that:
export XAUTHORITY=/home/<your name>/.Xauthority
Now if you will execute a graphical application command in acpi handler it will start, but it will be executed by root user. ...
As expected, recompiling libinput allows to use different acceleration profiles for touchpads (flat or adaptive scheme).
Then just creating an X.org configuration:
$ cat /etc/X11/xorg.conf.d/99-apple-magic-trackpad.conf
Identifier "Apple Magic Trackpad"
It seems the XQuartz X Socket is named /tmp/.X11-unix/X0, at least that file gets created by XQuartz when XQuartz launches, but the DISPLAY variable contained ":0", so socat was forwarding to a non-existing socket.
When I use that file instead of ":0" in the socat command (socat TCP-LISTEN:6000,reuseaddr,fork UNIX-CLIENT:/tmp/.X11-unix/X0), I get no error ...
Here is an update... I've removed 10-radeon.conf and 20-nouveau.conf and created a 20-vid.conf for all video configuration here. Just easier to keep track of what I'm trying, etc. At the moment I appear to have radeon recognized in the logfile without errors but the screen is blank. Previously when it wasn't working there was a cursor in the top left ...