Here's an unlikely theory. Please understand that I am putting this theory forth not because any evidence indicates this is probable, but because I cannot stand the idea of another beautiful innocent life taken. I just need to believe this could end with Aliayah's "safe" return.
There have been cases where parents have conspired with relatives and/or friends to hide/conceal a child and have some complicated scheme whereby (this is the idea anyway) information leading to the whereabouts of the child would be brought forth, sometimes through a lawyer, and then some substantial reward will be collected and later shared.
I am thinking of the case of Shannon Matthews, who was drugged, concealed and held for 24 days by the uncle of her mother's boyfriend (seriously, truth is so much more twisted than any fiction). The promise of a reward was the entire motivation behind this kidnapping.
http://www.guardian.co.uk/uk/shannon-matthews-kidnap
I am not saying I believe this is what has happened to Aliayah, but I have to believe a lesser evil is even a remote possibility.