I don't believe that the comment you're quoting is correct. YMX's point about Ansem possessing Riku was supposed to explain how they knew everything...
YMX basically says to Sora that Riku being possessed by Ansem was how Xehanort knew everything that would happen in KH1 ahead of time. Him trying to...
Yes, the Adult and Child timelines are in the same universe. The two timelines are the same up until OoT, and the split in the timeline is caused by...