Woot! We got it working and here are some lessons learned.
Lesson one this extension has issues with the windows store music be app due to not the location of extension files not being were they should be
(This may be why I've had issues with other Music bee extensions
Thanks for helping me out with that as I didn't know it would matter until now.)
Due to lesson one, i installed the standard x86 installer and decided to try following the normal setup to see if it was ONLY because my other music bee was the windows store app, but I had the same issues as before.
Finally I tested the provided fix and it's working.
I moved the listener dll to the same folder as musicbee.exe and it is now running as expected. (I think, I'm not actually sure what "expected" is, but it mutes my computer's music bee and just plays through the chromecast. Takes a hot second to spin up, but it works and that is more than I had before.)
Thanks so much for your help. Hopefully my experience will help others too.