After reading that newspaper article about SD going after people with an ax, I am extremely concerned that SD has alot to do with Baby A's disappearance.
You have a man that has been arrested on domestic violence and swinging an ax at other people at one point. There are apparently drugs involved. Whether or not mom is on drugs is unknown but it sounds like the family may have been somewhere partying the night before. The baby wets her pants and they have to take her home.She may be cranky, crying and disrupted mom and dads plans for the night.
SD may have been very angry (sounds like he has a very serious anger problem) and flew off the handle on little baby A. Little baby A he knows is not his "real" daughter and a perfect storm of possibly alcohol, drugs, lack of sleep, lots of kids, 2 babies on the way and SD loses it. Hurts baby A/ maybe something happened to baby A right after they leave or within next hour. SD and mom don't know what to do - either she is injured very badly, and passes away during night and mom awakes to find her passed, but baby has bruises all over her from might before and she can't call police because she knows what happened and has already had her kids taken away before.
Or baby A is fatally injured right after party and SD and mom trying to figure out what to do now. So sometime in night SD takes Baby A somewhere, then comes back and goes to work like nothing happened. It's even possible that mom was not there when baby A was hurt - but finds out next morning. At this point mom has to go along and make it look like baby A disappeared because she knows if she doesn't go along that all her babies will be taken away, and she is afraid of SD because of prior abuse.
I hope this is not what happened but something in my gut points to SD's anger at the root of this,..