I too thought ECU with my Glow Plug fault, as with yours, if its not the Wiring or Relay, what's left ?
But I'd never seen anything like the big negative voltage spike before and didn't know how that would come about.
Indeed that goes over my head too, like a giraffes gonads. A negative voltage in a DC circuit is best achieved by having your meter connected the wrong way round. In your case we see -20v in a 12v Circuit. My guess would be along the lines of a Collapsing Magnetic Field, only the wrong way round !! ?? Which doesn't really add up or make much sense to me either.
Your Scope trace shows the Negative Spike at the "Control" wire and the LED you set up at the Starter Solenoid - The "Load" side of the Relay, Both playing up ?
Yet the Whole point of a Relay is that the 2 sides, "Control" and "Load" are kept separate ??
One theory is that with the relay energised pins 30, 86, & 87 will be hardwired to the Battery + terminal so not so separate after all, in this application anyway,