IIRC the DL rule is that for sufficiently long connections, the bag cannot be checked through. There's another range of connection times where you have the option of getting the bag or not. However, check in at UIO is likely to be handled by contract agents so YMMV.

He normally won't have contact with bag check agents at ATL. After exiting customs, there will be an employee screaming at people to go to the bag recheck place where workers will just load bags that are already tagged onto the belt.