@Wonderful and Sephiroth But in BBS, Sora did it subconsciously as a newborn and when he was only four years old. Him being the only one to have done this just seems odd. If others are capable, how come no one else has done it? The series treats it like Sora is the only one who can.
I also don't like the idea of the Fortellers being one of the Xehanort's either, but both Demyx and Larxene were rather weak compared to the other members and had very childish, immature, and annoying personalities. Neither of them seem like the "final villain" type who Xehanort would choose to become one of his vessels. After reading that other theory regarding the 13 Hearts of Darkness, I feel like the seating may have more to do with a chronological order of who was chosen than who specifically is the Xehanorts.
As for the Lingering Will comment, it's pretty much been accepted among a large portion of the fanbase that the Lingering Will is Terra's soul. I'm not sure where it came from, but I always assumed that it was his soul because of that. I initially argued against it, but I came to the conclusion upon hearing that Master Xehanort sent both his heart and soul to replace Terra's and thus Terra's was cast out into his discarded armor.