Your piers are labelled correctly, but not your connection points.
The mainspan (span 7) center vertical strut is member U1-L1. The next strut south of there is U2-L2. The next strut north of the center strut is U2’-L2’.
Your cantilevers and trusses are generally correct, but the two center cantilevers are drawn a little generous. In the proper labelling scheme, the pier 6 cantilever only extends out (towards midspan 7) to U6-L5.
I’ve looked very hard at a triggering failure in the southern approach. Over and over again, I have to rule it out, at least as the most likely sequence.
Primary reason, as you note, the pier 6 superstructure would have rotated, top towards the south, if span 6 sagged midspan. That would have lifted the south end of span 7 well up above the horizontal plane of the deck, and we simply do not see this in the video.
If it happened that way, the top chord fractured north of pier 6 almost immediately, before any (assumed) visible deflection took place in span 6, and before said deflection could induce (actual) visible elevation of the span 7 south end.
I won’t rule out a triggering failure at the pier 5 crossbeam/endbeam rocker assembly, given the history there.
But the video does not visibly support such a sequence, and therefore, it’s not the first sequence in my assessment of the likeliest possibilities.
Leni