I have an update for anyone interested...
I'm still working with support, but my issue is partially resolved by uploading an app to the Meraki cloud vs. hosting with an external link. The app does eventually install on the target Mac, however Systems Manager will give a "failed" message, and the app will appear in the "missing" category.
M_agent logs on the target Mac will give an "install complete" message in the log, but will fail to ECHO "success" or "failure."