So I finally cracked the Performer open again after letting it sit for almost a month.
The schematic showed that all pin 7's are connected together. So I traced pin 7 from the IC on the board in the C section to where a tiny spring was inserted and soldered in place. There was a trace that led to all the remaining pin 7's on the other IC chips. The trace looked fine but the spring in pin 7 on the C section looked like it was barely soldered to the board. I just applied more solder and bingo, the C notes no longer held an infinite sustain. Fixed!
So to summarize in case anyone else runs into similar issues with their Crumar, I started out with the bottom C being completely dead and the remaining C's having a much prolonged sustain in comparison to the other notes no matter where the sustain slider was positioned. Swapping out the IC's in the C section with an IC from another note section corrected the dead C. That turned out to just be a bad connection. The C sustain issue was corrected by applying more solder to the spring leading to pin 7 on the IC in the C section.
Big thanks out to rhino for the advice and encouragement throughout this repair!