Lots of interesting feedback. Thanks everyone.
To answer some of the questions raised:
The major engine work done was replacement of the head gaskets. Like many Subaru EJ25 engines, this one developed an external oil leak right on schedule at 100,000 miles. The engine wasn't consuming abnormal amounts of oil (usually about 1/2 qt for each 6000km) but the bottom of the engine would be coated in oil and she would mark her parking spots with a few drops after each trip.
I never thought that the metallic spheroids on the ground electrodes could be deposits from the oil. I thought it was actually the platinum pucks breaking down but upon closer inspection of the photos it definitely could be deposits. The oil used during the tenure of these plugs was Maxlife or a blend of Maxlife and Synthetic Maxlife. (The shop that did the head gasket replacement used Motul 5w-30 but the plugs only saw that oil 630km before getting pulled.)
The issue I was investigating when I pulled the plugs for inspection was what felt like an ignition miss whenever I would let the car creep forward at idle while crawling in slow traffic. Initially I thought perhaps one of the plugs was damaged so I replaced them with the car's old original OE plugs which I keep in the trunk as emergency spares (NGK FR5AP-11). This didn't resolve the issue; if anything it made it worse. After that I measured the ignition leads with a multimeter and found an open wire. A-ha, found the smoking gun...or so I thought. Changed the ignition leads and the problem persisted! Finally did an idle relearn procedure and that did the trick.
The gasoline used is normally Shell or Chevron 87 or 89 (on hot days.) Once in a while I run a tank of premium to get the additional cleaning additives. I do not use any additional fuel additives. I'm reasonably sure the engine is stock and unmodded though I can't be 100% sure since I'm likely the car's third owner.
WobblyElvis, thanks for that super informative link. I sure hope that it's not aluminum from the top of my pistons melting. The shop that did the head gasket replacement would have had a good view of the pistons when the heads were removed however and they didn't tell me about anything unusual there. Fingers crossed!
Slacktide_bitog, I actually just re-installed the Denso plugs as they idle smoother than the stock FR5AP-11 and I wanted to try every permutation of plugs and wires. (This is Bitog afterall!) I'll run them for a few weeks while I order some new plugs. Given the rapid erosion I've seen on this car as well as my previous Impreza, I'll probably go with something else with a thicker center electrode. Perhaps Autolite XP, Champion PLatinum or Iridium, or maybe go back to Bosch Double Iridium plugs which I was running prior to the Densos. (I don't run the stock FR5AP-11 since they are only single platinum plugs and I think a DIS/Waste Spark systems should run at least dual platinum.) The choice of plugs will depend largely on availability though and I do need to get something quick since, as many of you have observed, the center electrode doesn't look like it will hang on for much longer.
For the curious, here's a link to my old post showing how NGK G-Power plugs fared in an Impreza I used to drive (similar kilometers):
https://www.bobistheoilguy.com/forums/ub...re_Electrode_Er
You can see undercutting of the platinum center electrode there also.