1) There may be micro stratches caused by either scrubbing with a wire brush or walnut blasting. For me there was nothing particularly visible using either methods.
Note that as mentioned the hardness of walnut media is nothing compared to the metal. I also blasted a piece of plastic (much softer) to see what occured and the answer is not much... It was smoother. Bear in mind valves are hard, hard. Im not sure of the Vickers number or material but they move at the speed of a bullet in one direction and then come back again many times PER SECOND so they have got to be pretty damn hard.
2) Unless you put some deep scratches (0.5mm +) in it it really wont make a difference. Consider the smoothness (or lack of smoothness of the surrounding casting) which will create turbulence in the intake charge. While this turbulence may be planned for it will make turbulence caused by small sctratches in a valve pretty small relatively speaking. Bear in mind that this is a forced induction engine too.
Bottom line. I think either will work and unlikely to damage the valves.
Unless you start pouring cleaner or walnut media into the cylinders I have no idea how you would mess up downpipes or turbos. And you would have to be an idiot to pour carb cleaner and walnuts into your cylinders.
2008 E92 335i