From your description of your daughters injuries I'd guess that the car seat was that Cosco model that closes around her waist and belts across with the vehicle belt. This type was the subject of an expose' recently and may not restrain a child. If the child is small or does not fit snugly, it may come of the seat especially in a roll-over. In a frontal collision, the head and upper body whip forward, then back with greater freedom of movement than a harness type seat and if any impact is involved, the head is much more likely to be injured because its movement is so unrestrained.
When it came time to choose my sons car seat, I spent an hour going over car seat aisle and was shocked at some of the crap it is legal to sell. Some seats looked nice but if you felt the back to see how comfortable they would be, or looked under the fabric cover, you would find 2/3 of the back was unsupported. Others were too easy to envision various types of failures, or just seemed cheap. I settled on the fisher price model which was the only one with a steel tube frame around it and a five point belt. The plastic was padded with stryofoam. My son once unbuckled it himself, and was able to reach under the cover and dig out foam with his fingertips, but otherwise I consider it to be an excellent seat. Some seats seemed fancier or were more expensive, but this one looked well made and the safest. Because of various issues I have heard of over the years, I would never consider Cosco anything but thats just me.
I'm glad you both came out as well as you did, and best wishes for both of you.