Re:
Need Fuel system help 1991 300SD, Bosch CSI

You are talking about a 300SE, are you not.
There may be a crime scene here, but the Bosch system is CIS. Not CSI.

You have the Accumulator and FPR identification correct.
The FPR however, is downstream of the FD, in the return circuit.
As a check for fuel flow in the FD,
1) Loosen the six fuel injector line fittings in the top of the FD.
2) Bridge the fuel pump pins in the pump relay socket so that the pump will run continuously.
3) Push down on the air meter plate. Does fuel flow out of the loosened fittings?