Depending on what you mean by True Black and White, If you mean utilize only 2 colors to render the image the reduce the color depth two 2 colors. If your talking about a black and white picture type deal, then grayscale is what you want. As it will use varying degrees of gray to render shadows and light. preserving the depth in the picture.
I am not aware of any method of changing black and white with one click. There might be some plugins available that do it, but you are going to have to google for them.
For your NEXT question:
You want to grayscale a layer and then not have it be
affected? This does not make sense.
Or you want to duplicate a layer, and then grayscale the opriginal layer resulting in 2 layers one in grayscale and the other in color?
If it is the second one, just go to the layer pallette and duplicate your origianl layer, and then grayscale the new one, that way you'll have 2 layers,one which is in color, and the other in grayscale. To alternate between the two, you could clikc on the little eye icon on the layer pallete and it will enable or disable whichever layer you are currently looking at.
----------------------------------
Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.