This is a common issue, I've repaired them before by bridging the connections you have identified with an external piece of wire.

The relay as you say is grounded via pin 10, the only plausible reason I can think of is an immobiliser, which the ECU's don't have/support anyways.

I was told the cause of this issue is people starting them without the battery earths connected, not sure what truth there is in this.

It's not overboost/hard cut protection, as this is achieved by cutting fuel injection.

Martin