The reason there is no documentation for API is because I’m not sure myself what would be the best approach. I’m experiementing myself with what I like the most.
One thing you can’t do is accessing SQLite database directly because Manager doesn’t store data in normalized scheme. SQLite database is used as a glorified file format, not as a relational database.
I think custom reports would be the best approach here. You would send JSON definition of custom report and get back custom report in JSON format. But this is not yet supported.