Charlotte, read your thread, but will answer over here if you don't mind. Can't be doing with the dad brigade saying how your abusive ex may have gone away and changed. Has he fuck. What he has realised is that the mat has been withdrawn from under his feet, and he can't carry on having every thing his own way, so hfesse has changed course to a new plan which hmm, let me see, involves him having everything his own way, in a different context.
The difficulty is that you need to separate your relationship with him from that of dcs. For dcs, contact is good, though youngest is struggling. I think there are a couple of points
Consistency. He can't give you a list of dates he can't do. You need dates on a schedule he can reliably do. DC need commitment to whatever contact is agreed. I would be very surprised if 50-50 comes around, but you do not have to agree that in mediation, anyway. You can agree a schedule he can do (suggest EOW and a midweek night, which still gives him flexibility for his job, whilst consistency for dc), and monitor for six months, you can schedule another mediation to review arrangements etc.
Additional dates to make up for what he misses are a no-no. DC have activities, friends etc, which need to be arranged, also dc need consistency for wellbeing and security. He should commit to schedule, if he cannot make it, he does not mess everyone around, he sees dc when he is next due to see them. You cannot arrange your life around his work commitments, nor should dc.
Your 3 year old. If she is getting distressed, would argue for reduced contact till she is older. Again, the point that you review in six months. It is better long term for her to have a secure relationship with her father, than x amount of hours per week.
For mediation, prepare, prepare, prepare. Know what you want, know where you are prepared to compromise. Have it written down. Know what you will not accept. Do not get deflected on anything about the marriage breakdown, emotional blackmail, or anything. Only respond to things about dc and what you are there to discuss.