Relative to the third part of your question, OSIRIS-Rex will provide new data that may be helpful in more accurately estimating the future path of potentially hazardous asteroids by characterizing the Yarkovsky effect.
The idea of the Yarkovsky effect is a body in space absorbs light from the sun. This absorbed energy is re-emitted as infrared radiation. If the body is spinning or tumbling, this re-emitted radiation is in a different direction. Although this photon pressure (think light sails) is extremely small, it doesn't go away. So over several decades, an asteroid that we calculate (without taking the effect into account) will miss Earth by, say, 30,000 miles, might not miss at all!
Bennu (aka RQ36) was chosen partly because it is so black. It has an extremely low albedo, reflecting only about 4%, meaning it will absorb about 96% of the Sun's light. This provides a worst case that OSIRIS-Rex can observe, providing useful experimental data.