In step #3 in your post #5, what happens if you use citric acid bath in place of tap water?
Good question - I didn't try all CA washing, but substituted a portion of the wash with 0.3% CA. Nothing happens color-wise, it acts as a normal clearing agent giving whiter whites (particularly post-ferri toning later) with some loss in Dmax when compared to all tap water process of the same overall duration.
Here's another twist to the story, as if one was needed - if I have pre-treated with CA as a part of the initial wash, then there is little or no "toning" with citric acid afterwards. Same thing if I used sodium citrate or EDTA. May be this is a clue.
The acid toning occurs with other acid too - oxalic, acetic, and sulfonic. The last one moves the color to greenish hue.