I know you rarely like what I have to say, but on this I am going to say it anyway.
You are WRONG not to tell her father. This goes beyond custody issues. You can do JAIL TIME for this. You can do up to one YEAR in jail and face fines of up to $25,000.
Her father NEEDS to know the full extent of the problem. Half of this problem is because she uses the custody issue against you and you allow it. I don't CARE what kind of a jerk he is. Maybe she NEEDS more of someone being a jerk or mean to her to get her to do what is needed. Yes, that IS harsh. It also is what it is.
As far as her living iwth your mom, I seriously doubt her dad will agree and you NEED him to agree. The more you hide from him, the less he has to deal with and the more that he can use against you. Not only are you allowing him to not see the full range of her problems/challenges, you are setting up a very negative relationship between them. Yes, he probably has done his share to contribute and I am sure the stepmom isn't so fun to be around either. BUT the more you HIDE from him and hide the problems from him, and the mroe you support her view that dad is 'awful', the more you create a very bad dynamic between them. in my opinion if she is old enough to refuse school, she is old enough to tell her dad about it and to face him and the judge in court.
I think you need to realize that right now, if he does not get notified about this very very serious meeting, you could lose custody with-o him doing any filing at all. I seriously doubt the courts would just remove one child or that it would be possible to get them back in the near future, if at all. Not only will you not get child support, you also will have to PAY child support. The judge CAN remove the children based on this attendance issue. You kids could even end up in foster care esp if you give the impression that their dad isn't interested. Not wth Gma, foster care. Where MANY children are abused in every way. Would that be preferable to facing up to your ex with all you have hidden?
I can honestly tell you that if my niece was treated this way by her mother, we would ALL be pushing for a change in custody, not just my gfgbro. And I would support him, even with all the things that have gone between us. Because he would (and does) get his daughter to school every day, and do all he can to be a good dad. My exsil doesn't like what he does, but he doesn't liek what she does either. in my opinion you are handing your ex full custody and child support on a platter by not including him. Yet if you notify him (and can prove that you did) and he doesn't show up, it is one more mark to your benefit. Espw ith as serious as this is, not showing up would tell the court a LOT, but not being told tells them just as much about YOU. And it says worse about YOU.
To be frank, given your DUI and other recent problems, you need to do all you can to please the court. Right now things don't look very good from the outside and a big part is that you have kept ex in the dark. If he truly ignores thing or won't show up, then you have NOTHING to lose by notifying him, and everything to gain.
I hope your therapist can help you see this from the court's and school's and DA's point of view.