Oh my god. Well, everyone has already said anything i would add. Great advice and perspectives. That said...on the topic of legal action. I would push hard and fast. Like Kuulbns said contact your child protection service. this shit has GOT to be accounted for and prevented. They should have done waay more than they did at the daycare. Your child should have gotten medical screening, a papertrail started and you should still have lip marks on your ass from all the kissing they should have been doing. Also, they should have informed the parents of the kid that did the biting. That is not acceptable behavior and desperately needs attention.
Move forward with your own actions after the protective services have become involved. It will give you more weight to push around.
Someone should be fired for sure. Why the hell was a toddler left unsupervised with your baby?
Your poor baby. That pic a week later looks like she was in a lot of pain at the time.
I hope she feels better now.

Beautiful child by the way!

peace.