I have a theory as to how Matthew and Mary will get together. It seems apparent that Matthew is continuously put into harms way and from the previews one can assume he will be injured in some grievous way. I think when he comes back, and needs to convalesce, his fiancé will lose some of her shine for him and Mary will step in guided by her true feelings. At last they will be together, although in many ways he will never be whole again. This is mere speculation, but one, as a writer, I feel as inevitable.
This is just speculation from a writer's point of view.