It is good she is getting back on track so take it anyway it comes. Go to the co-parenting class. Maybe ask to meet with the provider once by yourselves first so you can see what this is about and how you were betrayed by mom and give your side of story to provider.
Starting Pre-k is a big change for many kids and takes a few weeks to get into schedule so that is not so uncommon. If she is not listening to teacher or big outbursts make a behavior chart at home and tell GD that if her teacher says she was good today , she can come home and do xyz (food, snack, game, stop on way home at x) but if she doesn't then she can not do it. Stick with it. After a few days of rewards, tell GD that she needs 3 happy faces and then she can get xyz. Up the anti as she starts settling in.
Keep documenting dates/times mom says inappropriate things to child. Do you have a GAL for child? Tell GAL. Is the court going to entertain a custody petition since is has not even been a year?