Supposedly, the Cascio tracks are co-written by Eddie Cascio and James Porte. A demo, or guide vocal, may have already done by James Porte before 2007. If that's the case, what is the need of a guide vocal from Michael?
Take Fall Again for example, before Michael's demo, a full demo of the song was recorded and sent to Michael. Michael liked it enough to agree to record a demo. The version we all have is a one-take version recorded by Michael while he's watching his children played. What a fantastic job he did!
Take You Are Not Alone for another example, R. Kelly recorded a demo and sent to Michael for his consideration.
My point is that none of the Cascio tracks were created by Michael from scratch. Most likely, demos of the songs were recorded before Michael's stay in NJ in 2007. If that's the case, why would Michael record another version of guide vocal to be replaced later.
Also, we are talking about Michael Jackson - the perfectionist who put 200% of himself in everything he did. As humble as he was, he had a lot of pride in his works. It's out of his character to record half ass works (yes, even the demos are to be replaced later). If he recorded one half ass demo, okay, it's possible. But, 12 half ass demos. Sorry, I really have to wonder.