The short version of my question is:
Assuming the definition of the minor scale in Kostka's Tonal Harmony textbook (Eighth edition, chapter 4), which is that:
there is, in a sense, one minor scale that has two scale steps 6 and 7, that are variable. That is, there are two versions of 6 and 7, and both versions will usually appear in a piece in the minor mode.
how is then a step vs a skip/leap defined? Is, for example, 6 to raised 6 (raised here means in comparison to natural minor) a step, or does step mean the scale degree has to change: e.g., from 6 to 7 (I assume the latter is correct [more details below])? Is "7 to raised 6" considered a step?
Some more information from the respective section (example numbers from the Kostka book are given in brackets):
In an example (Nr. 4-2) the note material is written out for the root E as: E(1),F#(2),G(3),A(4),B(5),C(6),C#(raised 6),D(7),D#(raised 7),E(1). This example is not called a 'scale' by the book, and there is a bracket above 6 and raised 6 (same for 7 and raised 7). One could thus infer that these are the options you have for scale degrees 6 and 7, rather than it being a 10 note scale (which of course would be a strange way to view it).
Now a section follows stating that:
Melodically, the most graceful thing for raised 6 and raised 7 to do is to ascend by step, whereas 6 and 7 [meaning natural minor scale degrees] tend naturally to descend by step
Then some examples are given (Nr. 4-3 and 4-4, from Bach's Well Tempered Clavier), where, e.g., the sequences "7,6,5" or "raised 7, 1 (above previous note)" occur (which is perfectly in line with the above statement). Then it is stated that
If a 6 or 7 is left by leap instead of by step, there will generally be an eventual stepwise goal for that scale degree, and the 6 and 7 will probably be raised or left unaltered according to the direction of that goal.
and then some examples (also from Bach - Well Tempered Clavier) follow that gave me some trouble:
One is the sequence "raised 7,5,raised 6, raised 7, 1 (above previous note)". It is stated that the first note (raised 7) is left by leap (which would more precisely be a skip i think?) and thus the question if the 7 gets raised or not depends on the "eventual stepwise goal". Now this is indicated to be the 1 at the end of the sequence, thus the initial 7 is raised. Enclosed in this sequence there is a "raised 6, raised 7" (going to 1(above previous note)). Why is the raised 6 not the eventual stepwise goal for the first 7 (which would thus be unaltered)? Is it because unaltered 7 to raised 6 is not a step? Does this mean that it's only a step between 6 and 7 if both are raised/unaltered?
While writing all this down things got a lot clearer, but the question on what exactly the conditions for a step are, are not completely clear to me. The book also does not give much explanation for the statements regarding this example. I, from what I tried to figure out, would assume that indeed both 6 and 7 have to be raised/unaltered the same way. This would however forbid the unaltered 7 in a sequence "7,raised 6, raised 7, 1 (above previous note)" since it is not leaving by "step" (since one is unaltered and the other one is raised) and thus would target the 1 and would be raised (because of the 1 at the end of the sequence) even though for "(raised) 7, raised 6" it is then also not a leap/skip which in the example given initiates the conditioning of the raised/unaltered option to the "eventual stepwise goal"? This seems to me, to be a special case not mentioned in the book.
The section in the book makes it clear that the mechanism explained above is not a hard rule (in fact some examples from famous pieces are given where this "rule is broken"). Nonetheless, it points to the given example and makes the statements i described above, so i assume there is some logic to it.