Reporting.GetConnection(sPAW_URL).Get("tm1/" & sDatabase & "/api/v1/" & endpoint & "?" & params)
Correlates back to the Excel user's defined set of connections, which is likely the environmental factor. It does not work in full isolation of the connections as configured.
In terms of the REST VBA facility in the PA for Excel API, I would agree that it is very much a 'raw power tool' in it's current state; we're always interested in making improvements here - Our work for the API activity is primarily feedback/demand driven. I'll be sure to bring this into those discussions as we iterate.