I came back earlier than planned. Do I get the unused days back?

Quick answer

If you booked directly with Movepark: no. After check-in there is no refund, even if you pick up the car before the booked exit date, because your space was set aside for the whole period. Before check-in, on Flex and Superflex you can change the dates under "My bookings", and if the new period costs less, the difference is refunded. If you booked on the partner lot's own site or WhatsApp, the rule is the one they stated when you booked.