I just skimmed the last few pages. Your compression on the effected cylinder is crap. Johnbigdog hit the nail on the head with "the next step is a compression leak down test". Absolute must unless you like throwing thousand dollar bills at a fairy tale. A leak down test will isolate your problem-air leak heard in the induction side=intake valve issue. Air heard leaking out the exhaust= exhaust valve issue. Air heard leaking inside engine/at open oil cap=problem with the rings/piston. The difference is the cost of pulling the head and fixing it vs cost of tearing down the block or replacing it. A good tech always does the homework first before throwing parts at a problem. The picture of the plug coupled with the description of it dripping with oil tends to make me believe piston/rings but you need the leak down test on that cylinder.
Another thing (not particularly about your car) is the fascination with valve coking on these engines. If someone can explain it to me please do. On old port injected engines it was a issue since the fuel was sprayed at the valve and much of it would be absorbed/condensed on the crap built up on the back of the valve and cause mostly cold engine/idle quality issues. Our 3.5L is Direct Injected so in my mind the only thing moving past the intake valve is air (forced induction air) and it really shouldn't have a huge effect in my mind.
Back to this vehicle, the fact that you had so much oil inside the induction system and coked on the valves also leads me to believe that it could be piston/ring related causing an excessive amount of blow by.
Keep posting, I'm really interested and you've almost got it isolated. The leak down test on that cylinder will reveal your answer-if it was improperly tuned before you bought it, that piston may have also spark knocked itself to death (read, a good look with a quality bore scope).