Great analysis. I am an engineer who has specialized in metal fatigue. I lived across the alley from the bridge while it was being built. (The houses on the alley behind me were torn down for the bridge.) I used to walk along the railroad tracks under the bridge while on a hiking route that included the upstream Old Stone Bridge when I visited the U (a little Minnesota lingo, here). So I have followed your reports with interest. Please include me on your ping list.
I think fatigue played a critical part in this collapse, and as a result, find it hard to look away from the U6 gusset area.
At that point, the top chord goes into reversal from being in tension nearer the south piers. It’s carrying the upper end of the U6-L5 tension diagonal, and the upper end of U6-L6 also in tension. Further, its got the upper end of the L7-U6 compression diagonal.
There’s many opposing forces coming together at that critical point, and repeated loading and unloading of the span over decades of carrying traffic essentially have to focus fatigue inducing alternating and differential stresses there.