Re: Third Filter.
I don't know. It was an actual tattoo -- with an accompanying ritual -- that let Eyghon possess Giles and his friends. Not the same as a mark burned into flesh.
My memory is a little fuzzy because I sort of had Giles' soul inside me at the time. He could explain it better. But Eyghon didn't need the mark to possess someone unconscious or dead. That's how he got Giles the second time. So how sure are you that getting rid of the Mark of Cain will fix Dean?