Yup. Throw the original engineer under the bus, despite the fact that if the folks, including the latest engineers to approve the ongoing renovations, knew the thickness of everything and still went ahead and put large uneven loads on the structure.
Going to be a whole lot of arguing about these points for a long time.
In the report it states that the gusset plates should be stronger than the connected beams. Therefore, when bridges are reanalyzed it is standard practice to assume they are and not reanalyze the gussets.
Also, not unusual for an structural design problem to remain hidden for years before failure. The plant I work at had an original design error that let lose after being in service 35 years.