In my experience, negative values in dark grainy areas is very common. This is something compositors are used to dealing with and generally speaking these negative values are not problematic (unless a compositor is inexperienced and does not know how to deal it properly).
Yes exactly. In my humble opinion, negative values in grainy areas below a certain threshold should not be considered as out of gamut colors or even colors at all. If the max pixel value of an rgb triplet is 0.000006, should this really be considered a color?
Another question I have been wondering about is the question of exposure agnosticism. How important is this feature and why? The shadow rolloff approach to reduce the invertibility problems caused by very high distances in dark areas of grain does introduce a small change to the behavior of the algorithm with adjusted exposure. However this change is not excessive or even significant in the testing I have done. I would be curious to hear other opinions on this …