Not against the rules, but you might find more knowledge in a sub with more specialization on the film industry.

Your question does bring to mind one film, although not sure if it fits either of your criteria of 'development hell' or being discovered by 'film historians' necessarily: A Film Unfinished (2010) Ger/Isr dir: Yael Hersonski. Film website: http://www.afilmunfinished.com/

Probably not what you are looking for, but here are some examples that come to mind:

The Thief and the Cobbler is a film that went through development hell and was pretty recently recombobulated to a close approximation of the creator's intent.

In terms of films "lost" to development, there are numerous unproduced scripts. However, these scripts were never "lost" in the sense that we couldn't find them.

The only script I can think of that was once thought to be lost and later found is the original copy of The Passion of Joan of Arc.

You might also be interested in the ongoing project to recover lost Dr Who episodes (described here).