Yeah the 75 bumpers are pretty weak. Explode into a million pieces in an accident (experience). The factory plastic welding between the bits is haphazard, I think I braced mine with lots of brackets screwed and glued on.
My car also has rust where you describe. In heavy rain I get damp A pillar carpet so I think that may have caused it.
1988 Alfa Romeo 75 2.0 Twinspark
1994 Toyota Celica GT-Four Group A
1992 Honda Civic Si