Interesting thought, it would definitely play into the whole spirit of the Silent Hill mythos that the people and the creatures in this world are manifestations of one's own personal demons.
To put another spin on that idea, while Maria is the vamped-up version of Mary, maybe Laura is the child version of Mary?
The part of her that wanted to live, and the part of her that's messing with James in childlike ways in an attempt to show him that what he did was wrong (like startling him with the piano, and locking him in a room, which in a subtle psychological way, shows him what it's like to feel trapped, like Mary felt trapped in her own skin).
Just a thought! XD