Regarding the Nightfall asteroid:

1) it's been seen by observatories in the USA, which implies it's visible in the Northern Hemisphere.

2) Could the asteroid be seen from the Southern Hemisphere? How about the equator? Or within a few degrees of the equator?

If it is on the plane of the ecliptic like the planets and most asteroids it can be seen from pretty much anywhere. Extremely far north and south it would be close to the horizon which makes viewing difficult. If it is coming in from the north then it wouldn't be seen from the south but still from the Equator. Polaris The North Star for example can be seen slightly south of the Equator due to refraction in the atmosphere but no farther south. And that's almost due north only off a couple of degrees.

In some ways it doesn't matter where the asteroid is because the amateur astronomers to are the first line of defense with regard to tracking asteroids share their info with a world-wide data base. They e-mail pictures they take and it is shared over the globe.
For more info go to The Planetary Society's webpages: http://www.planetary.org/about/press/releases/2012/0315_Asteroid_2012_DA14_Discovery_Enabled_by.html
A piece of trivia: To get to Hawaii from Tahiti, the proto Hawaiians used Arcturus to navigate by since they couldn't see Polaris.
r/
Artemis

Thanks! I think I can use this in the fic and fudge things along. (along with other generalized handwaving and technobabble. )

Interesting trivia about the Hawaiians! Of course, anybody that would go on the Pacific Ocean in an outrigger canoe has got some serious courage.