Re: haunted GUM (update)
I am so out of the loop on this one, having gotten to read all of the posts yet except Marek's, Tom Sobota's, and your recent one. So I really apologize if I have stuck my neck out where it doesn't belong this late in the ballgame. I am just hoping someone will come up with a reason for your quandary so I'll share what my experience was with tonal inversion.
I have gotten tonal inversion when I was doing experiments with lemon juice, making the mix more acid. I would suggest mixing up completely fresh gum from powder, using it right away, and in place of water use pure lemon juice in your mix, either with dry dichromate or dry gum, and see if you notice the acidity affecting it in any way in comparison with one that is made with water. Marek explained that the lemon juice actually is a reducer in the equation, and perhaps there is some sort of reducer happening in the layers that are less exposed.
I never use sun exposure, only UVBL and consistent at that.
Another thing--is your sizing. If at all possible, if you could get your hands on glutaraldehyde (here come the list criticisms) and use that as a hardener for your gelatin, that would be a good test control to see if it is the hardener that is the issue--if I remember you are using chrome alum? I wonder if the alum is hardening post-sizing.
That's all my (probably unhelpful) advice for today but I do hope you get somewhere with all the advice you have been given to date on this issue....
PS in one workshop this summer a student got severe staining with black, and finally we realized she was printing on the unsized side of the paper :)
Christina Z. Anderson