The incorrect use of an abbreviated callsign by the TRACON sector radar controller to issue a descent clearance to N434PA and the sector controllers failure to detect that the clearance had been read back with the full callsign by the pilot of N304PA. Also causal was the pilots failure to question a clearance that descended them below the published Minimum En route Altitude (MEA). Contributing to the accident was the failure of both the Center and TRACON controllers to properly respond to the aural and visual Minimum Safe Altitude Warning System (MSAW) alert.