"This is the key moment."
Translation:Este es el momento clave.
You don't need un hispanohablante nativo to tell you that this makes no sense. Clave is an adjective modifying momento, you can tell because it changes the kind of moment to a specific type. Spanish syntax places modifiers after their related nouns, so clave is in its correct location following momento. I am not sure what you mean by it being used for emphasis and why that impacts word placement. To reverse the order changes the sense of the phrase to nonsense since momento is not an adjective. The closest approximation is that this is a "moment of keys".
Adjectives sometimes do appear before the nouns they modify in Spanish. See for example https://www.spanishdict.com/guide/adjective-placement
I tried "Este el momento critíco", but DL did not accept my answer. I somehow felt "clave" or "llave" is too literal to be used here, but apparently (and curiously, actually) it has a similar metaphoric meaning en Español as well. At times, I find similarities between English and Spanish metaphors astounding.