So I finally managed to take off the air cleaner case and the ECU as well as move cables out of the way sufficiently to take off the manifold covers and take a look at the glow plugs. The good ones look like this (please excuse that pine needle there):
But the bad one looks like this:
To me this looks like electric erosion from an arc, but I might be wrong.
Is this something that the vaunted special tool the Subaru dealerships have can deal with ? To my unscientific mind, this looks like a sure case of needs-to-be-drilled.