Nightfall was not a threat to the Earth. Anything that gets between the Earth and the Sun, which Nightfall did since it blocked sunlight, is already past us and can't turn around and come back.
It could spiral in if its trajectory was at the correct angle, but I admit that that's a bit iffy and would require some serious computation to check if it would work.
In relation to the Earth the Sun is downward. Think of the Sun as the ground, and Earth as a high flying plane. Nightfall would be between the plane and the ground. Not a problem.
Not so. The Earth has its own gravity and there's a point at which it becomes stronger than the Sun's -- or we'd all be falling into the Sun! A better analogy would be to adapt the relativistic rubber sheet model: think of the Sun and Earth as two valleys; the Sun valley is deeper than the Earth one, but something that moves past the "lip" between the valleys is going to fall into the Earth, not the Sun.
Yes, the physics of the episode is dodgy -- but then, what can you expect from a space agency that comes up with such a stupid way of dealing with the problem? A little lateral shove at a far enough distance and Nightfall becomes just another NEO -- a slightly big one, but no more trouble than any other.
Phil
