Why does the MiG-25 have silver-coated panels?
As you have pointed out yourself, the Silver is there because it is highly reflective. The panels around the engines were coated with silver to prevent the heating of the fuselage around it. According to this document,


  The engine bays each incorporated heat shields coated with a 30-micron layer of silver (5 kg/11 lb per  aircraft), which absorbed 5 percent of the heat and represented a more cost-effective solution than gold or rhodium, though these  metals were tested.


The panels basically trapped the heat within the engine bay, which is then cooled using boundary layer bleed air from drain slots between the air intake sides and airframe sides.