@KRegan There are a fair number of missing details from the 3rd Party Module instructions, especially when it comes to registering the device. I ended up jumping over here:
https://developers.google.com/assistant/sdk/guides/library/python/
and just going through the Google instructions in detail. Once you have the demo software working (audio only, without MM integration), it was fairly easy to sew them together and get it working on the mirror as well. I used my Jabraspeak USB speakerphone, which registered as card 1, device 0 on both the speaker and mic.
Have a scratch pad of some sort handy (OneNote, Notepad, etc) to track the various keys you will generate along the way. Once you get everything onboard, the sequence is boot the Pi (I have pm2 set to autostart the Mirror), connect vis SSH then start the virtual environment (source env/bin/activate), drill down to the pi folder (cd MagicMirror/modules/MMM-GoogleAssistant/pi) and start the assistant.py file (python3 assistant.py).
I took the better part of a weekend. ymmv.