hmmmm so much info...
1. the dark spots are most likely corrosion (duhh)
2. The pits is interesting. of they ARE pits in the chrome(depressions) then they would be very unusual. Usually these are actually bumps (little metal zits...) these are common and caused by the crappy metal underneath "out-gassing" as they age. The only solution that "I" am aware of is to de-plate, clean/polish, then re-plate. the issue is that because the metal has not changed, the issue will return (in a few decades) This is a VERY COMMON issue in cars built before the 80 when metallurgy caught up and under plating got started.
3. The biggest issue with just cleaning the spotted areas is that most likely the rust is underneath the chrome as well. (see above)
4. I use either "krud-kutter" or naval jelly on the rusted areas, wire wheel gently to clean then polish or plate.