IMO that thing needs to come out.. I don't think a vavle guide/seal can trash a plug that bad in 2 weeks. You can leak it but i really don't think it's going to change the final result.
I might do one last thing.... change the plug wire to that cylinder with one from another and see if the problem follows the wire, or stays with that cylinder... I have had wires that will ohm ok but still not work under load...
All in all i think it needs help..