Closing the loop for archival purposes – The red plastic clutch within the shaft coupler is wrong.
Thanks to @Luke and Brandon Lee (via support@carbide3d) for figuring this out!
Correct coupler with larger ID and the red plastic drilled through (Brandon’s photo):
Incorrect coupler with smaller ID and red plastic not drilled through (my photo):
Support is getting me sorted out quickly with the right part. Thanks, guys!