Tom, the answer to many of your questions is 'No one knows'. Perhaps it would be best if you focused on one question, formulated into a sufficiently precise statement (by doing a few minutes of self research to get the basics) and asked them one at a time.
For instance, how one gets particle generations out of string theory is somewhat involved, but often quite beautiful. There can be a very precise answers, but as usual depend on which type of vacua you live in. As an example, in heterotic string theory, the number of particle generations (minus the number of anti generations) is a topological invariant.
So suppose you are given the exact metric, and look for smooth continous changes of the geometry (induced by say quantum effects). The massless spectrum might change as the generations and anti generations pair up to become massive or vice versa, however the actual total number is always fixed, implying that the net number of generations in some corner of the configuration space is stable.
You might get a different mechanism elsewhere. But what is weird is that the *structure* and many if not most qualitative features of the physical world seems to fall out of the mathematics, even in unrealistic models. I mean why should a theory of strings, know anything about particle generations?