I tend to agree that IRS could be one of the reasons why they haven't disclosed the true extent of unreleased material. It goes with their line of argument that MJ's image and likeness was next to nothing, he hadn't released or toured in years, no big brands wanted to associate or sponsor him......
Another interesting speculative tit bit, I had read somewhere was about the preference to release unfinished demos, instead of completed tracks from the OTW, Thriller and Bad era, where the songs were composed and written by Mike, was to keep Quincy getting royalties for those songs as a producer. I would like to know from people who know better if there is any truth to this ! Even on Thriller 25 only songs that were Mike's are the ones that were remixed. (Thriller, Baby be mine, Human Nature, LIML, PYT weren't remixed. They had Will I am remix the other PYT that was Mike's.)
I think Quincy has the first right of refusal to re-work any of the released songs. So as long as Quincy is around and behaving like Quincy, I don't think the estate is gonna have it easy with songs from that era.
If i remember correctly, in the estate vs Q royalties dispute, it was the music released on This is it, Q had issues that the music was reworked without his permission. The estate argued that music wasn't reworked and that they merely used Michael's home demos before Q worked on them.
I may have over simplified it. Please feel free to correct me if necessary !