For most of your questions it is dependent on how bad the pitting is vs. how hollow the razor is. A singing hollow can't take much sanding. A half hollow can take a good bit of sanding. And a true wedge can be completely reground. For a singing hollow 600 grit, full hollow 400, Half hollow 320, and true wedge 80 grit.

As to what grit to start with use double or greater than the lowest grit you think will be necessary. It is better to take longer at a high grit than to destroy a razor, with to low grit.

How I hold the razor is based on the shelf in the Library's: Hand sanding a blade.

Jonathan