It sounds like you are going through a lot. I hope you are able to find the best ways to help your daughter.
I have not heard of insurance covering pediasure, however I'm pretty sure you can buy it at Costco and it is quite a bit cheaper.
We had 2 preemies who are little, even when we adjust. They eat well and both my husband and I are small boned and thin, so we try not to worry about it.
An alternative to ensure would be to make higher calorie smoothies or something like a breakfast cookie or even carnation instant breakfast mixed with whole milk. The point is something that has a lot of calories and nutrition that is easy to get down. I don't have a lot of experience with this with my kids, but when I was recovering from anorexia years ago, I had to drink a lot of ensure...and eventually we found alternatives I liked better. Cheese, dried fruit, vegetable oils, nuts or nut butters, avocados, coconut and similar foods are healthy and full of calories. Wild salmon, free range organic beef, etc are also full of healthy fat and protein.