Ok, now we're cooking! @tekka, thanks for explaining some of the details that I was missing. I'm sure I read that before, but I guess I swapped "gateway" with "controller" in my head, assuming that the sensor node would be assigned an id by the gateway.
I placed the following in my sensor node's sketch:
#define MY_NODE_ID 2
This avoided any errors, but I still wasn't seeing the correct message come through MQTT.
Not sure if it was required, but I ended up having to add
#define MY_PARENT_NODE_ID 1
to the sensor node's sketch, and
#define MY_NODE_ID 1
to the gateway node's sketch. Once I did that, the temperature readings started showing up in MQTT!
Thanks for the help everybody. I still have quite a bit to learn, but getting a working sensor sending data all the way through the gateway to MQTT was the biggest hurdle to really making progress on implementing these. THANK YOU!