WOW! That is a terrible install... FWIW, I think your plug problem is just the O-ring leaking around the valve cover for #5. I notice that was suggested by Terry & is a good call. One of the plugs on the Zetec I have looked exactly like that after the timing belt change. Fresh seals for the valve cover fixed it, no problem. Plug looks clean like the others now that there is no misfire issue.

Must be that jumbly-wumbly thing happening again.