Hello Rob,
The Sandbox, as you point out, does not provide any actual report
data. The ReportService in the Sandbox is hopefully useful for when
developing or testing code to schedule and download reports.
When it comes to report processing, I'd recommend that you develop
and test against a report or set of reports specific to your accounts.
The easiest, and free, way to generate such reports is to use the
AdWords web interface to set up a report with the same options of your
choosing, and then download the XML of version of that report from the
web interface. Everything other than Structure reports should be
available via the web interface. You could of course generate the
reports that you'd test your processing code against via the
Production ReportService, but that obviously incurs a charge of 500
API units.
Once you have a set of reports that contain all the columns that
you'd like and some actual statistics relevant to your account, just
reference one of the reports locally during your development or
testing process when it comes time to parse your report.
Cheers,
-Jeff Posnick, AdWords API Team