I think part of the confusion that arises in this issue comes from the fact that the US statute refers to something becoming fixed. That has nothing to do with photographic fixer or any stage of development. Instead, it addresses the issue of determining whether a work that may have copyright attached to it is in sufficiently complete form for it to make sense that copyright be enforced with respect to it.
The language is there to make clear that each stage of a work that leads up to an appropriate level of completion doesn't get attached to it all the trappings of copyright protection - authors are permitted to make and discard their drafts.
In essence, the development of film stops the clock on any further changes. Until then, the photographer is free to add to their work by exposing the film to additional light. Developing film has absolutely no practical or legal effect on the legal and moral interests created when the film is exposed, save and except for the fact that once the film is developed, the photographer cannot change that which he or she is entitled to control the copying of.
An undeveloped image is not yet capable of being copied, so all the enforcement tools in the Copyright legislation are not yet capable of being applied. But that has nothing to do with the rights to the image itself.
If you look for case law on this question, you will encounter several diffulties. The primary difficulty arises from the fact that you don't need to have the negative in order to own the copyright in an image, and because it is such well settled law that copyright arises at the instant of exposure, and any future handling of the film or prints is irrelevant to those earlier created rights.
The secondary difficulties arise from how popular the Vivian Maire case is - the search tools are flooded with it. The probate court there quite properly (IMHO) dealt with the issues of copyright in exactly the same way for the images on the developed film and the images on the undeveloped film.
The tertiary difficulty is that much of the case law on undeveloped film arises out of child pornography cases, and the question of whether possession of undeveloped film bearing child pornography in latent image form constitutes possession of child pornography - it does, but that likely turns on specific wording in the charging statues.
In the US, photographic images have specifically been accorded copyright protection since 1805 as I understand it, and probably had that protection more generally under earlier versions of the legislation. The US inherited copyright protection as part of the English law that applied before 1776 and was adopted by reference at the time of Independence. There are 240 years of copyright experience in the US and during a lot of that time people were taking photographs and benefitting from copyright protection of their work. The scenario referred to in this thread is not new, and any questions of law raised were answered long ago.