It's next to impossible to know at night.

The only thing you have visually is direction / angular velocity.

And if you're on a collision course the other vehicle will appear motionless, with no change in direction.

If the airliners was approaching faster than the helicopter it could have been approaching from behind. How large is the field of view of a UH-60?

"How large is the field of view of a UH-60?"

Massive, you're practically against the glass. Visibility below is more obstructed than visibility above or to the sides.

From the radar tracking, they appear to have been at least somewhat head-on.

(Also at a low altitude in a large city ... lots of lights.)

I wonder if there's a support element that would have obstructed the field of view over a narrow angle. This has been the cause of automobile accidents when cars approach at just the correct speeds to keep the other vehicle behind the pillar at the side of the windshield.

Yes.