Not an expert, just a newbie with some experience with scouring and weak calves. On mother's milk I suppose she got colostrum? Is the scours watery? Barring the premature part which I have no experience with, I suggest using kaopectate (or Kaolin Pectin at feed store) to stop the scours. If it doesn't stop with kaopectate, it will stop with immodium liquid (don't use immodium if you think the scours is anything but a digestive upset, immodium will kill a calf with other types of scours). If she gets too weak to suck you will have to tube her. Don't wait to tube like I did. I was afraid to learn but there is a tube will a bulb on the end that makes it impossible to tube into her lungs. Dehydration is the number one cause of death in very young calves. Get electrolytes (probably more important than milk right now)in her and stop the scours. Good luck with Missy. I don;t know what her chances are but at least she has someone giving her individual attention.