PBL,
Replying to your latest post first:
As far as I understand, for example, FullWings's answers, they would be:
1. Descend. Reason: conflict avoidance with unseen traffic.
2. Descend. Reason: conflict avoidance with unseen traffic.
3. Climb. Reason: RA advises to do so.
The difference between scenarios 2 and 3 is, however, minimal. Yet the avoidance manoeuvre is exactly opposite. (My bolding)
And the unseen conflicting traffic does not go away.
Yes. Nothing very surprising here. Does the absolute direction of the avoiding manoeuvre matter if it resolves the conflict? In your scenario No. 3 you're assuming that the traffic you can see is the one that has generated the RA... That may not be the case, especially as "It's night, so you have little range information, just direction." The information presented to you also depends on the type of TCAS installation and what modes you have selected; TCAS is much less accurate in displaying bearings than distances, so the actual traffic could be in a significantly different direction. Also, if you can't see your 2 o'clock traffic and it isn't on TCAS then it's probably far enough away not to be an immediate risk, unlike the traffic that is causing the RA.