Where did you get the idea that you need to change or alter the ladder to install a touch probe? I've installed probes on two Fanucs and never did anything with the ladders other then what you're doing by verifying the skip signal is working.
If you MDI a G31 skip move and while the machine is moving, touch those two wires together and the machine stops, that's all you need for probing. A working skip signal. The rest is either home grown Probing Macros or those from Renishaw or others.
Well... that and a machine interface and receiver that works with your probe. (You do not hook up your probe directly to that input, but to it through the probe Machine Interface.)
You might want to find a spare M code or two for turning on and off the probe, but even that doesn't necessarily mean altering or messing with the ladder.
I installed probes on a Fanuc OMC and 18MC
Look into the Installation Hardware and Installation Function manuals for you OiMD. You might be able to find a High Speed Skip Input.(That may be what you've already fond. )
I think the X inputs are generally the normal skip signal. That F one you found sort of rings a bell on my 18MC install, and that uses the High Speed Skip input.
It was a lot of research and work for me to install these probes, but eventually I got them both going. Tool touch probe too.
There's a lot of info to be found in the Renishaw Install Manuals and Software Manuals.