So far, it seems the most plausible explanation. That's what I thought, actually. Everybody else dropped out/weren't capable enough and little Lavi was the one to stand up for the Clan.

Hmm, that makes me wonder about the reaction of the Clan itself, and how they organized themselves in this kind of situation. Could the apprentices bail that easily? They were probably killed, as you say, when they tried to escape. It'd be pretty obvious, then, that the Bookman clan is quite a large organization with many members scattered around the world. Or maybe they do have a permanent, hidden location somewhere. And the current Bookman with his apprentice are the only ones let out to witness the world's events as the other members of the Clan remain hidden in said location, protecting the records and putting them into place.