The thing with LinuxCNC, is I don't want to fix it, I want to use it, which I think is it's biggest failing.
I'll admit I'm a windows guy, and it makes life in CNC alot easier. My machines make me money, so my priorities are a bit different from a typical hobbyist, as I want to fit something, and have it running with minimal hassle, rather than spend hours coding something that already exists in other software.
Have you found any manuals for your servo drives?
If you can find one, it should make your life a lot easier with configuring the drives for a different controller.
For USB/Ethernet, the PC is usually just sending trajectory data with he motion controller handling all the critical timing stuff.
It does mean there is a delay from changing something on the PC, to the motion controller carrying it out, but for most things it's not noticeable, although it does vary depending on motion controller and software. Some combinations can have very noticeable lag.
However when done correctly, it works just aswell as any other motion controller, and quite a lot of the high end controllers now use ethernet. USB is generally not used in industrial enviroments as it can be succeptable to noise problems, and is the only negative thing about the KFlop I can think of, but in several years I've only seen one person have a problem with it.
A major advantage is that if the PC does die, which is usually the most unreliable electronic part, you can simply plug in a different computer, load the required software/config for that machine, and away you go again.
Personally, I have a computer dedicated to each machine, with configs backed up to a memory stick, all the software required installed on my laptop, so if a computer does die, I can connect my laptop, load the latest backup, and be running again within a few minutes.
But anyway, keep the progress posts going. It's always nice to see how other people carry out retrofits, and how they solve any issues they have along the way.