Whether or not an asteroid hits us in the next 100 years is pretty much deterministic (non-random) and depends on the laws of physics. If a large asteroid is going to his us, it is already on the path to do so. The only uncertainty lies in the fact that it is too hard for us to predict what is going to happen over the course of 100 years. Of course it is possible that we will be able to deflect such an asteroid in the future.
Perhaps not. An asteroid in the field between Mars and Jupiter can collide with another, sending it on a trajectory with our orbit.
What's scary is that our recent near misses went completely undetected because the asteroid originated from path between Earth and the sun, making it impossible to see until it passed.
Not necessarily. An asteroid is capable of outgassing which could affect its orbit.