First, thanks for your review. I wish more people who enjoy the game made positive reviews since some guys play the game for 10 minutes and make bad reviews with almost no basis to do so.Once you've unlocked sandbox mode, is it possible to get to the encounters you missed because you were working on another angle? For example, I missed out on the car races / girl entirely.
As for your question, no. The sandbox mode works like an improved gallery. You can only repeat previously unlocked scenes. In the future, I will add exclusive scenes not available in story mode (there are a few already).