This still bothers me:
7:54 - Center - Descend to 2500
7:54 - PGW - 2500
All indications of time and distance that I can calculate indicate that the aircraft was losing height at around 500 fpm, from 3 minutes north of YSRI. At 12 miles to run the aircraft should have been at 2200' or better, but reported 1500'. Nine miles later, or 3-4 minutes, it had lost the 1500'.
It seems quite possible to me that the pilot may have reacted to the ATC call by stepping down at some point below the best descent angle, possibly due to an innate compliance with the normal requirements of the airspace as he was dealing with the mechanical problems.
These are only quick calculations I've made and I would be happy for someone knowledgeable to refute my hypothesis.