Thinking a bit more, the whole "soul splitting" thing could potentially work, actually: the two different iterations of de Rais clash so much that his soul actually splits into two: one ascends to the Throne, while the other is left behind as a Wraith.
Okay, now I have some questions:
The Throne of Heroes is outside of time and space, inside Akasha, right? Does that mean that though the Holy Grail calls them down one at a time, the Spirits' departure could be considered simultaneous from the Throne's point of view?
Also, Len is a Demon-succubus-familiar-thing that messes with dreams, and is rather powerful, correct? Would it be too unreasonable to assume that given a big enough boost, she could potentially make her way into Akasha?