Alas, the aforementioned collection is not mine to share, but a good sample is file XAB_0871.NEF from
RawTherapee vs Darktable: invert scanned B&W negatives
It seems that Auto Levels from RawTherapee does not use the full range of available values. Starting with XAB_0871: neutral profile, crop, rotate, negate using RGB Curves, Black-and-White with desaturation, Auto Levels with clip 0% gives min and max values 0.128 and 0.965. Correction with ImageMagick-s -auto-level gives pretty nice result.
I was hoping for usage experiences with other algorithms: CLAHE, sigmoidal-contrast, etc.
Sigmoidal-contrast seems to be especially interesting, as it allows to later restore the original image and to start over. Has someone tried to auto-tune sigmoidal-contrast parameters?