Hey guys
FWIW, I was able to make this happen, thanks to the crystal clear, step by step instructions.
I did run into a few hiccups, mostly of my own making.
First, I made a clone of my Indigo OS to experiment with in case something went horribly wrong. But instead of downloading the latest 10.13 installer, I used one I already had. Once the upgrade was done, I was able to delete the offending files, install the right ones, gave them permission to run, and things were going smoothly, with the Insteon USB modem.
Then of course, came the update to the latest version of 10.13. During the update, as mentioned, Apple re-writes the AppleUSBFTDI.kext file, and puts it in original location, which needs to be moved. The problem I ran into when trying to do so was there was already a file in the location indicated by the Terminal, so I couldn't move it.
I then tried booting into the new OS, and moving the file in the GUI, but I wasn't able to do anything with it because of a permission issue. So I booted into the old OS, and was able to delete it from the new OS drive from there. After that, I booted into Recovery Mode, moved the newly written file to the now empty location, re-started, and everything seems fine.
While I pretty brave about the terminal, my skill level is very low. In retrospect, while in Recovery Mode and working in the Terminal, I could have deleted the first moved file, or I could have moved the 2nd file to a different location, but honestly didn't think of either option at the time, and even after, had to look up the commands to do so.
A host of untried options. but at least at the next OS update, I've got a few things to try to make this easier...
Anyway.... for now, everything seems to be working as it should. I'm going to give it several days... maybe a week before I clone the new os back to the internal drive of the Mini I'm using for Indigo.
As always, thanks to both
Jay and Matt and
everyone else who has contributed to this patch. Here's hoping Apple can see their way clear to fixing this, and soon.