I've seen progressio used as a part in a story which progresses the plot, but is there a word for a moment with action and conflict that determines the outcome of the story, or a crucial time for a character to make a decision? Any related terms are welcome, preferably in Classical Latin.
The word you are looking for is gradatio, -onis, (f.), the Latin equivalent of κλιμαξ, used by Cicero, for example, de Or. 3, 54, 207,
Est etiam gradatio quaedam et conversio et verborum concinna transgressio et contrarium et dissolutum et declinatio etc. . .
[EDIT] This use of gradatio has been deprecated in comments. An alternative has yet to be put forward, possibly because none is known.
Any modern translation into Latin almost inevitably has deviations from classical usage — something which is often not fully appreciated. For example, a change of subject as the sentence proceeds may occur more often than a pedant might consider strictly proper. It has also to be recognised that style is one thing, and idiom another, and that it is sometimes necessary in some way to transform both — as in the present case, when a deficiency of vocabulary has to be remedied. In some such cases an existing word can be used, if the translator feels that it can be adapted sensibly. In the end, a successful translation has to be a compromise between accuracy and readability. Vitruvius uses gradatio for a series of (stone) tiers. Cicero, in the absence of a precise Latin term, adapts the physical idea—'steps' in the most literal sense — to a rhetorical use. I have proposed adapting gradatio to the result of a sequence of occurences, or what can be called in ordinary language a 'climax'. I repeat what I replied to one of the comments : if a more convincing suggestion were to be put forward, I should accept it enthusiastically.