B Does string theory provide any additional insight to quantum mechanics?

I only peek at string theory occasionally to see how it is going - last I looked: "no"... though that is the short answer,
Probably string theory is best thought of as a meta-theory: a kind of scaffold on which a theory of everything may, eventually, be constructed.
Studying string theory can get you an understanding of symmetries etc that crop up in QM ... that sort of thing. Just like higher dimensional geometry can lead to understandings in 3D geometry. In a kind-of abstract way. For instance, it may be possible for someone to find insights into how QM and GR do not work together.

At best we would expect a new theory to provide insights into the old one in much the same way that the heliocentric model for the solar system provides insights into the geocentric one and Newtonian gravitation provides insights into Ptolemy's epicycles.

 

Take Bosonic string theory for instance. You start with the classical sigma-model action, find the equation of motion and write down the mode expansion of the solution. Then you evaluate the Poisson brackets of the modes and perform the usual canonical quantization (so called "first quantization"). The modes follow the usual creation-annhiliation algebra of the quantum mechanical harmonic oscillators.

But in the process may you end up with negative normed quantum states. You get rid of those states (and maintain the conformal symmetry at quantum level) by choosing specific background spacetime dimension and normal ordering constant (for Bosonic theory they turn out to be 26 and 1 respectively). Finally you study the spectrum of the states in the positive definite Hilbert space so obtained (and it includes gravitons)

So basically, you don't get deeper insight into quantum mechanics. You just use it as an ingredient to make a self consistent theory in the hope of explaining physical world.

 

At this stage it doesn't, but more insight could provide more insight, just as GR is 'emergent' from String Theory.

So I cannot answer your "how" question :P