“So I understand that the destination of a Sakadagami Anugami will be just the same as if he were a Sotapanna; that effort does not NECESSARILY translate into a better realm than the human one after death unless Sakadagami-hood is attained (for even Sotapannas MAY attain a deva existence – the difference of course being that for a Sakadagami that deva existence would be his last in the kamaloka)
Is this correct?”
Not really. The progress cannot be measured just in terms of “change of bhava”.
– One may have not reached the next destination, but one may have covered a significant portion of that distance. That is an analogy.