Version 5.7 attached
I had to give up on full manual exposure, as you really needed to scan most reels at least twice, to handle under and overexposed scenes seperately. The 8-bit Gamma 2.2 encoding can't handle the potential dynamic range. But I don't like the drifting nature of the stock auto exposure, so I made my own crude solution. The exposure rules are only to decrease when clipping, or increase when at least a stop underexposed, all changes are fast, in half stop increments. So on a scene change it may change exposure, otherwise the exposure is somewhat locked. All exposure changes are using shutter time only, ISO is locked at 50 for the lowest noise (unlike stock FW.) Shutter speeds supported between ~1ms to ~16ms (about 4 stops of range.) This is significant reduction for the stock FW range, which gains up black film way too much. Note: The EV bias is no longer used.
I have improved to auto stop, which would trigger for completely overexposed segments. It now waits for 32 frames of completely blank images.
Histogram is now stable. Not that this is completely useful, it was helpful in debugging exposure rules, it means I can now render other things over the video preview image. Currently the histogram only runs during capture, not preview.
Final change, I disabled 3DNR, multi-frame noise reduction, which compares frames for "better" denoising. If this was implemented at 25fps, the video camera's frame rate, it would likely have been an excellent option, but it was running at the encoding frame rate (2fps.) The issue is the difference between frames is not sensor noise, but film grain, and it was removing texture and details from the shadows.