I don't actually dislike how young you made Marquis look, even if that wasn't intentional on your part. He's stated as being a very young man before being sent to the Birdcage, so if he was in there for about ten years(If we assume Amy's 16 or 17 by the beginning and we know her to be 6 when he's detained) he was there for about 12 years. I like the idea of him being in his mid thirties by the time Amy gets her tats.
So, I guess maybe it's a scalpel that's being held like a shiv? Showing that even when Amy believes that she is irredeemable, she is still clinging to hope. Or that by using a scalpel as a shiv (an instrument of healing as a weapon) she acknowledges that there is no right/wrong to powers themselves, only to how they are used,