Looks like the north end of the bridge was just responding to the collapse, from what I read. Pics of the south side show lots of twisted metal, and it seems the bridge deck is slanted sideways. Does this mesh with your theory?
Yes.
If the pier or vertical members of the steel truss failed, the bridge would tend to drop straight down.
If the bridge swayed to either side during the fall, it is more indicative that the diagonal bracing between verticals failed first.