Hello K..
You have a great deal of information here to help you out. While you don't need mine, some things I would like to offer may come as comfort.
My son is 5th percentile by the CDC 2000 growth charts that all pediatricians should be using, including yours. He has been since about 4 months. His pediatrician sent me to a feeding clinic over it, stating that my continued breast feeding was the source of the problem. She also provided some of the suggestions you have here: carnation instant breakfast added to foods and liquids, pediasure, increased whole milk intake, butter prepared foods, even fried foods.
I was distraught. My son has always been a good eater, including preferring fresh fruits and veggies, and healthy foods. So I did research.
I found that for this age, about 1000 calories daily is what is required, and that like puppies, at this age, children will often swing from thin and leggy to pudgy with each growth spurt, and in between.
More significantly, I also found that the WHO and UNICEF are redo-ing the growth charts for children from ages 0 to 5 years, setting children who are breastfed until 1 year of age as the standard. When I plotted my son on the preliminary chart they had released, he went from 5th percentile, to nearly 50th.
The current growth charts were produced by milk companies. Up until 2000, those charts didn't even include breastfed babies. The update required by CDC and released on those charts in 2000 only includes some breastfed children. I don't know whether you breastfed your daughters or not, but the implication of the WHO and UNICEF changing the growth standard this way is significant.
We live in a country in an overweight crisis. The percentages of weight related diseases are skyrocketing in every age group, as is the mortality of weight related diseases. I would pray for you to consider that first and foremost before beginning a diet with your daughter designed to increase her size. The growth charts are just a number. With that in mind, the best advise I have for you, and have seen here offered to you, is to look at your daughter for what she is: if she is eating well, seems healthy and happy and to be growing, even if it is not at the chart standard, then thank heaven for a healthy, happy growing baby. Worry about a number when it actually means something.
After more than $2000 spent for an evaluation at a feeding clinic, I was told by a pediatric feeding specialist, dietician, pediatrician, and occupational therapist, the entire evualuating team, to completely disregard the suggestions above from the pediatrician, that my son's eating habits and consumption were just fine, and that he has a very concerned mother.
I believe the same is true for your daughter.