I guess I'm missing the point of this, so I'm probably looking at it wrong.

If you're saving multiple ancestors in ancestors_between why not save all of them?

And if the goal is to access the info for all of the ancestors, what makes it more likely than average that your ancestors aren't on the same layer as yourself (i.e. that e.g. half of them aren in ancestors_between)?

Because, if a fixed ratio (50 or even 10%) of an arbitrary node's ancestors is at the same layer, isn't complexity still the same (only reduced by a constant factor)?