M.M.
Wow Melissa...You've got your hands ful!
Only thing i can think of right now is them cabinet closures they look like a Big U and have a locking placer to set where it needs to be. you can also let her know that you are going to tighten the bottom part in order for her not to get out and you will make her uncomfortable until she stops!
you can take her to fire station and put alittle scare in her too.
The fire fighters willt alk with her and let her know that is it important to keep it on and teach her a few things to be happy in the seat. Maybe you can try to get her mind off her seat by giving her a book, toy or even place a Portable DVD player in car for her to watch something too. We have 2 one in each car and my girls love it but of course my daughters do not try and get out neither. Hope this helps! Call your local Fire Dept. and ask them can you come down and they can talk to you directly but her as a indirect way to stay safe. Only good things for Our Children.
M.