Since I'm fairly new to the board, I couldn't post a reply or question in the DEV threads, but hopefully the solution I had to take will be helpful to others.
I started from a Knox-free MD7 build on my S3 and just followed the root and then MK5 ROM update directions... all of that went cleanly.
The fix was to * Update the Modem to MK5 * as well!
After updating the modem software to match the ROM, a quick reboot later and the activation worked now and, after a couple of minutes (and maybe another reboot or two) it updated the PRL and Profile and both 3G and 4G LTE were working again!
Looking back now, though, I realize that I got carried away with playing with the new ROM after Step 7 of @jdsingle76 's directions and I failed to follow Step 8...