It will surely burst when it reaches about 100k ft. We attach a parachute to the payload to make it descend gradually. Other than that, there is no control over where it's going to land. However, because the payloads are small styrofoam boxes, they don't pose a very serious danger in the rare case that they land on roads. Flight predictors are really surprisingly good at simulating where the payloads will land, so it's usually not a huge surprise.

We'd rather be safe than sorry. So far, nothing really bad has been reported in the HAB community. It will leave a bad stint if something blows up on someone's face.

Regarding authority, we often file a NOTAM-Notice to Airmen (https://sites.google.com/a/mtlsd.net/citizen-science/ib1/not...).

The balloon is supposed to burst, the payload should have a descent parachute. It's a problem if the balloon doesn't burst and drifts into busy airspace.