Can I backfill payments for weeks before I started tracking a creator?

Partially. If you create a payment group with a start date in the past, the system can backfill base pay per video and posting counts because it can see when videos were uploaded (up to 90 days / 200 videos of history). Milestone bonuses can also be backfilled, but only for videos uploaded after the payment start date, since the system can verify that all views on those videos occurred within the payment window.