The one that fails is the fuel pump safety circuit - #20 like you correctly identified. I checked my notes, which don't need quite the squint as does the Bentley. The other pin, #21 runs the main relay with ignition on. The only function you'd lose by not supplying this with the ECU's logic is the power to burn off the AMM wire.

Unfortunately the fuel pumps have to shut off via pin 20 when ignition pulses fail-- not so easy to work around safely, although one could use the old k-jet relay to provide this sensing, I suppose.

Now speaking of squints... Here's the rub with the 561's losing fuel pump ground output. The #20 pin that drives the relay coil comes from the third pin from the left on this hybrid microcircuit in the ECU. The hybrid's design was changed to one that never seems to have that problem sometime during the production of the 561. All 951's use the newer hybrid and all I've seen have either one or the other. I haven't figured out how to identify from the outside of the ECU box which chip it has inside. If someone knows, I have a few here I can add to the data pool.

Hopefully I won't screw this up and get them reversed, but here's how the two chips can be distinguished. The first pic is of an old 561- prone to the fuel pump ground problem. (Actually I still find it amazing an external relay is driven from a transistor on this hybrid instead of a cheap off-substrate discrete unit.) You are looking at the "southwest" corner of the hybrid viewed as you hold the ECU in front of you connector down.