I would avoid the REST API for this and write a simple Extender business rule that queries the audit tables and puts that data in table(s) staged in your local RDBMS for reporting as needed. Reference the Extender rule in a Data Management Sequence & Step, and automate the data pulls nightly using Task Manager from the Marketplace.
You can learn about what data is in which table in the OS repository by going to the System tab, click Database, then Application Database, then Tables. Here you can see all the tables, their columns, and the data.
