Just speculating here
1) EB told her cousin she was "afraid to call 911" because CPS had already been called to the home. This leads me to believe that EB was more afraid of CPS finding out and confirming little Zahra was indeed enduring severe neglect and/or abuse. If EB was more "afraid" of CPS finding out than she was "afraid" of Zahra dying, she would be motivated to continue allowing Zahra to die (as would AB).
2) If AB was working for the past 6 months, I would then believe EB was in care, custody and control of Zahra for most of Zahra's awake hours. I bring this up because I am still startled by how much weight EB appears to have lost since the May 2010 hearing aide video. Therefore, I contend that if EB wasn't eating regularly - I doubt Zahra was eating well regularly (as I am assuming EB would be in total control over what and when Zahra ate during the daytime hours).
3) If previous CPS involvement was in or around May 2010, I would think that the only reason AB and EB took Zahra out of school to "homeschool" her would have something to do with a referral to CPS for medical and/or educational neglect (ie: EB following through with obtaining the free hearing aides) right around the same time period. Often when CPS gets involved with families, they will work with the schools and local community resources to help meet the child's needs. I personally believe the family moved and kept Zahra out of school so that there would be no eyes on this child. They could isolate Zahra from friends, family, and caring 3rd parties who might look out for the best interest of the child.
3) Systemic organ failure can look like someone is suffering from a virus.
4) No medical treatment, no schooling, inappropriate nutrition, punitative parenting (making Zahra run up and down the hill despite the pain, forcing Zahra to learn to run before riding a bicycle), and not to mention AB's statement that Zahra was "brooding" and when she is sick - he just leaves her alone. I am positive that he was not the "nurturing parent" during Zahra's cancer struggles and pain. I believe AB left all of the nurturing and parenting to his Mother and when he married - he left it to the evil Step-Mother (who had no emotional attachment to this disabled child).
4) After allowing Zahra to die, AB & EB went into major coverup mode.
5) Zahra's bio-Mom claims she found Zahra 3 days before she was reported missing. I believe AB found out ED was trying to gain contact with his already (or nearly) deceased daughter - he had to have an excuse for Zahra not being there should someone begin an investigation. Keep in mind, there were no other eyes on Zahra. She had moved from the trailer she had friends on the street and neighbors saw her. She had been taken out of school. We have no mention of Zahra playing with neighbor children @ the 21st street address or even being seen at the property by neighbors (to my knowledge). The only "eyes" looking for Zahra was her bio-Mom, 3 days before she was reported missing. Not even EB's Father, who knew of Zahra existence, was in contact with this family. I think this is the reason they even reported a child missing, otherwise they would have continued on with life, because no one was checking in on Zahra. (Maybe AB's mother might have inquired about her).
You have summed up my theory in better words than I could ever find!! Great post! I definitely agree with #5. I believe ED finding her daughter had a big piece in this puzzle, or the idea that someone (anyone) was actually looking for Zahra and they would have to answer to the neglect.