As an art historian (and picture cataloguer), these questions have rattled around the caverns of my mind for decades. The rule of thumb re the conventions of art history is similar to Barbara's comment.
That is, if an art work (ie work and expression in frbr talk) is used inspirationally by another "creator", and thereby takes on a new dynamic and artistic form, it is clearly an independent piece. Marcel Duchamp's "Mona Lisa" of 1919 is a case in point. Duchamp simply pencilled a moustache on to a print of Leonardo's work/expression , taking the practice of art into the wild terrain of Dada (born 3 streets away from where I am sitting in the old medieval quarter of Zürich) and creating a new work/expression etc. Sometimes, these referenced interventions are not as successful or striking as Duchamp's and we need to keep reminding ourselves to ask what was the INTENTION. If a "new" work was intended, then we accept that, reserving judgement. Many amateur "copies" fall into this category. Forgeries do not. In art history thinking, the INTENTION behind any "reproduction" is critical. As a picture cataloguer, I can perhaps apply similar logic: a photographic reproduction or a print of "Mona" is most usually an exercise in neutral (commercial!) documentation, not normally a new artistic endeavor. So, presumably a new expression of an already existing work/expression? A real example from daily work: in our collection, we have photochrom prints of famous European oil paintings, mostly Italian. These anonymous photochroms were clearly intended to show the bougeoisie what certain famous works looked like and enable them to be collected personally in this form. So what are they? New expressions of existing works? On the other hand, our wonderful "topographical" photochroms of old Europe before it changed forever after the First World War, are original works and expressions. The (also) anonymous photographers who supplied the negatives for these prints selected their views and took their photographs, thus making this category of photochroms works as well as expressions. ????
