Making Kate Immortal - a better way
The way Duncan activated her immortality totally messed up his relationship with her, as evidenced by her running away from him immediately after and then hating him many years later.
What he SHOULD have done was to sneak up behind her one day and do it while her back was turned. Then, when she revives, he could have "found" her and said "Oh, my god, honey, what happened to you? Are you okay?"
"Oh yes, dear, I'm fine."
Many years later...
"Duncan, dear, have you noticed that we don't seem to be aging at all? Just curious..."