Place squash and sweet potatoes and leeks in the slow cooker. Season chicken all over with salt and black pepper and place on top of vegetables in slow cooker. In a medium bowl, combine orange marmalade and remaining ingredients. Pour mixture over chicken and vegetables. Cover and cook on LOW for 6 to 8 hours or on HIGH for 3 to 4 hours.
Serve 4 of the chicken breast halves with squash and potatoes on the side and extra sauce spooned over top (reserve remaining chicken for pizza). Top with parsley and rosemary just before serving.