That is the total opposite of what Al Stohlman and the old guys that taught me in the saddle shop. BOTH needles go through at the same time, eliminating the possibility of piercing the threads.
I have done numerous projects that I doubt I could duplicate. That's what makes them all special. Bianchi and others turn out cookie cutter products, not my thing.
I do not sand the flesh side but if I am gluing anything to the grain side I sand or otherwise 'rough' it up. To give the adhesive more to bond to. There should be enough 'roughness' on the flesh side.