Since then, more discussions have ensued on the SeeSat-L mailing list. Chiming in were amongst others Ted Molczan, Jonathan McDowell, Allen Thomson and Cees Bassa. These discussions and new pieces of evidence provide a possible target area for the test, and if some of the things brought up are correct, indicate that the launch location, the trajectory and imaged part of the flight path might be somewhat different from my initial assessment (which as I noted was very rough and very approximate: there was a reason I didn't provide a detailed map)
First, Ted Molczan managed to dig up a Broadcast Warning to mariners for the south Atlantic (that I was not able to trace to a URL). The text:
( 090508Z SEP 2013 ) HYDROLANT 2203/2013 (57) (Cancelled by HYDROLANT 2203/2013) SOUTH ATLANTIC. ROCKETS. 1. HAZARDOUS OPERATIONS 091400Z TO 140130Z SEP IN AREA BOUND BY 09-18S 000-26W, 09-50S 000-32E, 12-03S 002-39E, 13-40S 004-09E, 14-09S 003-49E, 13-06S 001-56E, 11-05S 000-58W, 10-55S 001-05W, 09-56S 000-50W. 2. CANCEL THIS MSG 140230Z SEP 13
Ted speculates that the area indicated is the target area of the (dummy) warheads from the Trident. Indeed, it is about 7000 km away, well within the ~11 000 km range of the Trident missile, from the general launch area I deduced earlier (but see below). It would mean my launch azimuth estimate was off by 40-45 degrees (and closer to 130 degrees). And it could very well be given that it was a very rough deduction from observations from only one location, with several assumptions involved. To reconstruct it properly, you need observations from two locations, so you can triangulate.
The potential target area is in the eastern part of the South Atlantic, between St. Helena and the coast of Gabon and Congo. It is elongated and the major axis of the polygon might be indicative of the launch direction. In that case, the missile trajectory was approximately as pictured below (Red line: missile trajectory. Yellow lines: sightlines from La Palma for the range I astrometrically measured (21:17:08 to 21:19:42 UT): this does not include the earliest part where it emerged from the horizon as seen from La Palma. The grey polygon is the potential target area mentioned in above Broadcast Warning).
Meanwhile, the actual launch location is a point of discussion as well. In my earlier analysis, I interpreted two distinct events in the photographed trail as the moments the 2nd and 3rd stage of the missile kick in:
Jonathan McDowell has a different suggestion: he thinks these moments represent the MIRV bus and MIRV (the warheads) separations. These happen at much higher altitudes than the rocket stage burns. It would mean the object(s) were at a much larger range from La Palma than I deduced from my earlier notion it were the 2nd and 3rd rocket stage burns. It would shift the launch location significantly more to the Northwest (see map above).