Aldebaran getting Subaru's memories from a Book of the Dead/the Tome of Wisdom doesn't make any sense.
Reading a Book of the Dead doesn't give you the Authority of the owner of the book, otherwise Subaru would have gotten the Authority of Gluttony after reading Daphne's book in Arc 6.
We know that clones can develop an Authority, otherwise, Aldebaran wouldn't have risked using Ol Shamak on her (he said that Sphinx was at the brink of developing an Authority, but just a few chapters after that, she was slashed by the Yang Sword and forced to abandon Echidna's identity)
Al is 100% Subaru's clone. How? No fucking idea, but it doesn't make any sense for him to be a random person forced to read Subaru's book a billion trillion times with the information we have as of now