From the definitions & gists, it appears that the key difference is:
Potentiality: there’s a small chance that, with evolution, it could become true in the future. “This stock will go through the roof.”
Possibility: there’s a small chance it is true right now–without any evolution. “I hear that there’s gold in those hills.”
Is this the key distinction? Something else? Thank you.
I think you have a good handle on the distinction between the technical (Dramatic dictionary) definitions.
However, I think there’s a lot of overlap and the lines blur a bit when you’re writing a story and the distinction is more easily felt than logically understood. In that sense I think Possibility and especially Potentiality are terms where the English words are much better at conveying their meaning.
e.g. in a story you can have examples of Possibility that seem to apply to things in the future, like this example from The Princess Bride analysis:
Once the cure is ready, everyone knows that it probably won’t work, so they resort to hoping for a (possible) miracle:
VALERIE (to Max): Think it’ll work?
MIRACLE MAX: It would take a miracle. Bye!
So maybe your term of evolution is good because the idea here is that the cure’s chance of working is already fixed, and that’s what they’re discussing. But “it would take a miracle” could easily fit Potentiality as well; that’s why you can’t determine a storyform from just one scene. (although one clue here is that Probability fits as Symptom a lot better than Certainty)
potentiality is “is even beyond the realm of possibility and spends his time focusing on the greatest possible potential” where as possiblity “gives less weight to the single most likely explanation, looking instead at the whole range of known alternatives”.
That’s the difference I focus on when looking at the definitions. All possibilities vs the most likely potential.
I always think about them based off of their Dynamic Pairs. Potentiality is paired with Certainty, and Possibility is paired with Probability. Certainty means “this thing must happen, so I don’t have to worry about any alternate scenarios,” so Potentiality means, “This is an alternate scenario that could theoretically happen, so you need to consider it.” Probability means “I’m going to take the action that is the most likely to happen,” so Possibility means, “I’m going to take this action instead, even though it’s less likely.”
EDIT: Potentiality is recognizing that 1-6 are potential rolls of a die. Probability is knowing rolling 1-5 is more likely than rolling 6. Possibility is betting on 6 anyway. And Certainty is loading the die to roll a 6. 
Here is the difference:
Potentiality = likelihood*
Possibility = could
*see revision below
And you thought CORRECTLY. Good catch. It would be better if I said:
As you can see, however, is that it is difficult to compare them as apples to apples. The above uses two verbs and two nouns in my attempt to simplify. That is because there is a base difference BETWEEN the concepts of the items in a Dramatica quad, and when attempting to simplify and remain generalized, those differences become apparent.
NOTE: Depending on specific context, likelihood may be an adequate stand in for potentiality, but not in the context of all four items in the quad. In that case, probability has a clearer claim on the term.