My name is Mike Viney (@mike-viney) and I am new to myFOSSIL. I know this is an old thread but I find the discussion interesting. I think premise 2 that molds can only be made against the original fossil should be revised to include the idea that molds can also take the form of empty spaces inside organisms.
This original premise does work well for an external mold that can be formed, for example by a shell or wood limb that is buried and decayed leaving an open space. However, the organism itself can have empty spaces that may act as a mold, such as the decayed pith area of a Calamites stem or the inside of an empty gastropod. Material that fills these internal molds form internal casts or Steinkerns. The surface features of these internal casts preserve the negative impressions of the original surface. In the case of the Calamites pith cast, groove structures in the original stem become rib structures on the Steinkern. In contrast the external cast preserves the original contours of an organism’s surface features.