In the short term you are in for some hard times, but I think that if you act consistently and lovingly over the longer term, that is what the kids will appreciate and respond to. You can't change their mother's histrionics or what she tells them when you're not there, but they will know when they feel secure and loved and all the routine stuff you do with them will help with that.

With regards the toilet training, if communication is so strained at this point in time that you can't both discuss this with the mother in a calm fashion, there are family mediation services in each state that will help you with regards to getting an agreement between both parties on consistent parenting, and also negotiating access visits. Worth ringing to see how they might be able to help you. For what it's worth, lateness with toilet training can be a sign of stress in a child so you might just have to ride it out (but it would help if you knew what the mother is doing about it). Have you tried using the pull-up pants and getting a sticker reward chart going for every time she is successful with the toilet? Try to stay positive, she will get there.