2TR-FE: Pulled spark plugs, one is not like the rest

My 2.7L 2TR-FE suddenly began idling rough and I got a CEL after ~40 miles of driving. Mileage is 167k. I replaced the spark plugs today (unsure when they were previously done). Engine runs great now with new plugs, very smooth, and feels a hair more powerful than before. Need to return to the auto parts shop tomorrow and borrow their scanner to pull the code(s) that I got. In the meantime, I could use a little help "reading" the plugs I pulled.

The one from cylinder 2 looks different/more worn than the rest. Anything I should look out for or any ideas what might cause this? Already caught the slight rust at the tip that may indicate a headgasket issue. But what's up with the wearing through of the insulator and fairly clean threads? Cylinder 3 also looks gunky.

 

Those look like very used plugs and # 2 doesn't look like the rest with the black rings etc...
Anyway the gunk is probably from the spark plug seals which is part of the valve cover gasket set.

I'd give them a few thousand miles and then pull them when replacing the valve cover gasket and have a look at the tips then.

Video for the gasket replacement.
https://www.youtube.com/watch?v=Fzm_W5PUCbI