If new plugs got that bad, that fast, I would say the valve seals (oil seals) are SHOT (and or piston rings, but since one looks fine, the odds are bad valve seals IMHO)!!!! I can't see a rich fuel mix doing that in such a short time!!!!
If it was something like a weak spark, it would be weak on all six plugs IMHO.
Only two options, pull and service or replace the head, or on a temp basis install hotter plugs or antifouling spark plug extenders, and some heavy duty snake oil additives, MMO to free up stuck rings first (It could be a ring problem or a valve seal problem, or both), then a thick oil burning oil additive, like Lucas.
Compression test, Do one cylinder at a time, and pull the primary spark plug wire before cranking. With that much oil the test should not need oil addition, LOL.