It absolutely does work over the wire. I've done it, and a quick Googling shows plenty of others getting it to work. (Example: https://apple.stackexchange.com/questions/246337/apple-magic...)

Correct. When I first converted an old, out-of-support Mac to Linux, I used its equally old Magic Keyboard, connected by Lightning-to-Lightning cable, since I often needed to make entries when `bluez` and other associated Bluetooth s/w weren’t yet running. (Have since replaced it with a regular wired keyboard.)

It definitely does - that’s even how you can most easily first pair them since plugging it in will automatically pair the Bluetooth device.

Is that so? I have a Magic Trackpad with a Lightning connection and I was under the impression that it did work over the cable. Thinking about it though, I'm not sure I ever validated that.