1) If you are a performance nut, they are faster, but if you are using VB, you arent a performance nut. Dont worry about it =).
2)ZBuffering takes care of overdraw. When rendering alphablended particles, and you turn z writing off, over draw DOES become a problem. What you can do is transform all of the particle center points with the world and view matrices, and then sort by z coordinate.
3) When one vertex is shared between several faces, its normal is the normalized average of all of its shared faces.
4) Do you mean Normals? It depends. See 3.

Z.