LinuxCNC New user experience

yes

Where can I find the communication protocol between linuxcnc and the ethernet GPIO bridge devices that seem to be required ?

Actually it's way more than a "ethernet GPIO bridge", the MESA cards employ a FPGA to at the very least create the Step & Direction signals from the Linuxcnc timing info (thats the simple explanation).

I doubt very much if it would be possible to get hold of the timing info and convert back to gcode to resend to a USB controller.

there is no standard protocol i think,
but you can take a look here: github.com/scottalford75/Remora/blob/mai...ents/Remora/remora.c
and see how other projects handle this.

the source for the hm2_eth driver used to talk to Mesa ethernet cards is  here 
github.com/LinuxCNC/linuxcnc/blob/master...a-hostmot2/hm2_eth.c
These devices are very dumb devices.

Regarding motion, Linuxcnc converts the required velocity to a stepper frequency and sends it to the the mesa stepgens. The stepgens will then oscillate at that frequency forever or until told to change it (which Linuxcnc can do 1000 times a second).

As I said previously, Linuxcnc is the motion controller and is in full control of everything attached to the PC.