Update for anyone who might come across this later: I managed to get the keyboard and track pad working. From a helpful poster on XDA developer forums (whose original post inspired me to give this all a shot), I learned that I had snipped the red wire in the wrong cable. It was the red wire in the USB-A to Micro-USB cable that needed snipping; I had snipped the one in the USB-C to USB-A instead. The photo was taken pre-correction, so you can see the splice I put into the wrong cable.

Here are links to all the stuff I bought to hook up my Note 10+ to the Atrix lapdock:

1. USB-C multi-port adapter: USB-C end into the Note, and other end for connections to the lapdock
2. HDMI to Micro-HDMI cable: Male HDMI end goes into the multi-port adapter, female Micro-HDMI end connects to the lapdock connector
3. USB-A to Micro-USB cable: Male USB-A end goes into the multi-port adapter, female Micro-USB end goes into lapdock connector. This is the cable where the red wire needs to be snipped.
4. USB-C to USB-A cable: Male USB-C goes into the multi-port adapter, and the male USB-A goes into one of the lapdock's rear USB ports. This appears optional, just to allow the lapdock to charge the phone from its battery during use.

There's something wonky happening with the connection between my Note 10+ (running newest OneUI 3.0) and the Atrix lapdock. Sometimes I can plug it right in and start working no problem. Sometimes, the kb/tp aren't recognized and takes some fiddling, whatever combination of which gets thing to work I haven't quite figured out yet.