A previous post mentioned the sentence taken from DWDS
NN und etwa 90 m über dem Talgrund der Schwarzen Pockau, zu der er steil hinabfällt.
to which Herr Schölnast commented
The relative clauses »..., zu der er steil hinabfällt.« (..., to which it falls down steeply) and »..., zu der hin er steil abfällt.« (..., towards which it falls down steeply) are two different clauses. Both are correct and have a very similar meaning, but not exactly the same meaning. Note, that also this is possible and correct: »..., zu der hin er steil hinabfällt« and even this is correct: »..., zu der hinab er steil hinabfällt«.
Now, this struck me as odd, because in English the word "steeply" is never applied to the action of falling. Things fall vertically as a result of gravity, not at some angle. But hinabfallen appears to be applicable to the action of falling, as well as to the non-action of something (a slope, a trend, etc.) falling-off or inclined at an angle. So in English something cannot fall down steeply, as Herr Schölnast puts it, in English it can only fall off steeply. I was going to chalk this up to my ignorance of German, and assume that in German steil can be applied to an action of falling -- somehow. But then Herr Bodo comments
I consider this as an error in this context. To me hinabfallen would mean that the rock formation would fall down to the river, which does not make sense. (It would be possible for a loose stone to fall down.) steil abfallen in this context means that the rock formation slopes down steeply towards the river. I think it must be "..., zu der hin er steil abfällt"
So now I am really perplexed about how all the options Herr Schölnast cites for applying hinab adverb with hinabfallen could be valid if Herr Bodo's observation is correct.
My examination of Duden's definition of hinabfallen does not show any associated use of zu or zu ... hin, which suggests to me that the verb hinabfallen should be restricted to the non-action of falling off and that the action of falling down is better expressed with "zu ... hin abfallen". That would correspond more closely with English usage, it seems.