I was hoping that you would be letting me know that she is back to normal and doing wonderfully. I am sorry to hear that is not the case.
In a young puppy who isn't eating well, I would be very concerned about her blood sugar being a cause for the weakness that you describe. These guys simply do not have the reserves that older dogs have to be able to mobilize fat and get energy. From that standpoint, in hospital, I monitor the blood sugar and give IV fluids with Dextrose as needed to maintain the sugar in appropriate levels.
I know that you can't do this at home, but the way to supplement is with the pedialyte or ensure on an hourly or every other hour basis to try and keep the blood sugar up.
An alternative explanation is if she had a distemper virus infection that is now affecting her nerves. This can cause extreme weakness and collapse as well. I don't find this as likely, since she hasn't had the typical symptoms (vomiting, diarrhea, sneezing and coughing), but I can't 100% rule it out.
Please understand that these are educated guesses. Without seeing her and seeing what her blood sugar and anemia are, I can't say that it isn't her low red cell count causing the weakness.
I don't mean to say to you that the anemia isn't a problem. It is. The point that I was trying to get across (and failing) is that you can't do anything about the anemia at this stage. If the vets think that it is not severe enough to need a transfusion, then it should not be severe enough to cause the weakness that you describe. And short of a transfusion, you can't do anything to raise her red blood cell count except for the supportive care and feedings that you are trying.
Have you been trying to feed with a syringe? Watered down foods like the a/d and the Recovery diets can easily be fed through a syringe (and in some cases where we want to do everything, a feeding tube is an option).
As for her long term prognosis, it depends on the underlying cause. If this was Distemper, she may have long term problems. On the other hand, I have seen many, many dogs who have had Parvo as young pups do very well later in life. So, odds are, if she makes it through this, she would live a normal dog's life (maybe a little spoiled, but normal).
I hope this helps!
I continue to be available for follow up questions if you have them.
I hope that Lilly turns around soon,