Imagining The Tenth Dimension

There's some issue with the tenth dimension, though. Even though it does contain all different timelines for all different universes and treat it as a single point, it does not contain itself. So, even though we don't have any other point in the tenth dimension, it does not contain everything: Namely, not itself.

Also, something which bothers me with the 7th dimension: Why do we just assume that it is all the possible timelines for a single universe given its start? Why start? Why not end? Why not midway?