Hi,
The custom reports are not supported by AdWords API at the moment. To get the stats aggregated at the account level, you will have to use the
Account Performance Report. Also, using the API you can only download the reports from a client account and not an MCC account. To compare the stats during two date ranges, you might have to download two reports with the same report definition and derive the change and %change during the date ranges.
Please let me know if you have any additional questions.
Thanks,
Sreelakshmi, AdWords API Team