The question says it all. Simply stated:
Can one prove grand unification from string theory? What is the argument chain of such a proof?
The textbooks I read so far only appear to give hand-waving arguments.
The question is not about deducing the standard model. The question is about deducing grand unification. This means that there is a single large Lie group.
Can one prove somehow that string theory produces a unique (large) Lie group?