Well... this is an interesting dilemma. It sure would be hard to read with runovers. Ideally, a foldout would work here but if this is being prepared for publication, foldouts are impractical.
Personally, I don't like the idea of keying entries. Besides, a key must take up two cells, so I don't see how you would gain room here since "cb" also takes two cells. You suggest using "c" to represent "cb" but this will certain cause misreading. Sorry, this is not an option.
I would consider leaving out one of the columns. I would choose omitting the "an-3" column. The dividend shown in the introduction doesn't go out this far, so eliminating it from the illustration shouldn't cause a problem. You would state the omission in a TN of course. Do I hear objections from any mathematicians?
I like your idea about repeating the arrangement without the second separation line in order to draw the arrows.
Thank you for this interesting problem. Does anyone have a different idea?