Since I'd already spent lots of hours on two days with this issue, I decided it was probably time to just install the system again and start over. I'm well on my way to having the system back to where it was before all that nonsense started. No, I still can't connect the new Arteck mouse, but my keyboard and Logitech mouse are connected, and I'm pleased with that.
It's going to take a while to restore all my email filters, but I can do that over time, as needed. So, as of now, my Plasma 6 laptop is squeeky-clean, and configured as I like it. And I wish I'd just done this before I spent all that time on it.