I think your question goes directly to the heart of the question, "What is a photograph?" Is a negative a photograph, or only a step toward creation of the photograph? Or can it be both?
There are several ways to answer these questions.
1. Legal creation. According to copyright law and criminal law, the image is created when the shutter is pressed. The film does not even have to be developed. Copyright attaches at the moment of pressing the shutter. This became an issue in several cases, especially early pornography cases, when photographers had taken photos showing pubic hair on the model and were then arrested before developing the film. (The showing of pubic hair was the dividing line between art and obscenity according to the post office regulations and most local obscenity laws until the late 60's in the US).
The copyright in the negative continues to be important. As the creator of the negative (or digital file), I control who can print the negative. I have to grant the right to print to another. Of course, the final print is also protected by copyright, a separate and distinct copyright from that of the negative.
2. Historical definition of photograph. The most generally accepted historic definition of photography was the creation of an image, usually a positive, on a light sensitive surface which was fixed to make the image permanent. Historically, I think the negative was viewed as a step toward creating the final positive. Except when the photographer intended to show the negative as the final image.
3. The combination image problem. What about the positive created using multiple negatives? Here the final image is not created at the pressing of the shutter, but when printed. The final image has no single previous iteration before the print is created. Indeed, the negatives may have been created days or years apart.
4. The post-processing problem. What about photographs created with heavy manipulation? And no, it doesn't have to be digital. I created a series entitled "Sculptural Bodies." The negative was bleached, scratched and drawn on before a print was made. The final print was nothing like the scene in the studio when I pressed the shutter. And when I pressed the shutter, I knew I was going to manipulate the negative, so it is not analogous to creating a score by the composer and an interpretation by the musician. (The manipulation of the negative was a further step in the creation of the score using the score/interpretation analogy). Someone could come in and print my manipulated negative to their own interpretation, but not the unmanipulated negative.
My own solution to the problem: I have a stamp that I put on the back of prints that I sell. The stamp has blanks for the title of the image, the year the negative was made, the year the print was made and the process I used to make the print (platinum, silver, cyanotype etc.) The stamp severs two purposes, it helps me to identify the negative used (I file my negatives by year) and it discloses to the buyer both the creation date of the negative and the print, thereby avoiding most of the issues raised by your question.