Posting the solution for anyone else who runs into issues with their GX470!
What wound up working was choosing the "Prado 2010-2013 CD HIGH" option in the canbus settings, and making sure that the OEM clock display thing (above the nav unit in OEM configuration) is plugged into the canbus adapter when testing climate controls. The clock looks like minor / optional equipment, but something about it is important to making everything work properly.
I also got some suggestions along the way to keep the OEM nav display plugged in and tucked behind the dudu unit - thankfully this was unnecessary and the on-screen climate controls work about the same as the factory controls. The only hiccup is that they only display temperature in Celsius, so us Americans are going to have to get used to doing some math when adjusting temperature.
