Wow! Sounds promising.Argh, tried to post this and my computer went squirrelly, so let's try this again!
Just had a random thought! Karen told us in a chat that Adam only thinks he's 6000 because that is when he drank from the cauldron. We know that Adam is the only Fae to have a child with a human survive to adulthood (Circenn). We see a lot of similarity between Adam's favourite glamour and Barrons' physical description. Is it possible that Barrons is Adam's child from a relationship with a human woman before he drank from the cauldron? In that case, he would have forgotten his child, Barrons would have essentially been abandoned. He could have been given the elixir like Adam gave it to Circenn and thus have immortality, but the question is how much of his soul, if any, remains? It is interesting to look not only at Barron's heritage (Basque and Pict), but also that he tells Inspector Duffy in DF that he was born in Scotland, and Adam has always seemed to have an affinity for Scotland.
(No, not changing my overall theory about Barrons, this was just a thought that popped up today!)