I am so sorry you are having trouble with your baby. Before getting another baby to take care of, I would try to get this little one on track.

From my experience, the more I changed Maddies food, the pickier she became. Find a food you feel comfortable with and trying sticking with it for a while, see what happens. You may even want to mix safe extras to help encourage the appetite: carrot bits, cheese, yogurt. (Make sure you research safe treats for yorkies.)
As for the biting, that could be related to food or environment allergies. This may be hard to figure out. I would start with a limited ingredient food such as California Natural or Newmans Own.

Did I understand you are using 2 flea treatments? IF so Why? That seems to be too much, especially for such a small baby.