TCAS is a transponder based system. Warnings are suppressed at low altitude by design, and city lights do not interfere with it.
Runways get changed all the time for many reasons. Every runway at washington national is in a different direction, it would be a different approach entirely and not a last minute change. There is no evidence that the plane flew the approach wrong. There is ADS-B data for the full flight. Anyone can check the plane's actual flighpath.
Pilots can refuse ATC orders that are unsafe. The approach they were originally planning would have crossed the river and had the same risk of traffic.
The helicopter pilot seeing the wrong plane is a likely explanation. There were other planes in the area. The controller warned of the traffic. The pilot confirmed having the plane in sight.
The helicopter was flying at 300-400 feet. The buildings to the southeast are shorter than that. Those areas are 3-4 story sparse developments not highrises. The plane was above and descending. It would be seen against the night sky. There would be no buildings near where the helicopter would be looking for the plane.
Landing approaches are started from specific navaids. This plane was not off course because it was given a different arrival route than expected.
The airspace around DC is some of the most restricted in the world. Routes into the national airport are very tight with little allowance for error. Most of the routes come in over the river to avoid overflying government buildings, and the involved plane had a sharp left final over the river. The plane may have turned when the helicopter wasn't expecting it.
This is all speculation. Investigations into things like this are thorough. It is far to early to assign blame to anyone involved.