Is there such a thing as a “snap judgment” incarnation, where the need to incarnate is so pressingly urgent, that there is in a sense no time to plan, and such a life proceeds essentially unscripted?

This does not happen. There are indeed sometimes rapid reincarnations when the timing is such that it must happen immediately in order to match timelines, to be in the right place at the right time, to be on a par with others who might depend on that individual to be in their life as a functioning person mature enough and aware enough to play a role that is much needed in support of their life purpose and perhaps high-level mission. But there is always planning and the reason is, in the light, time can be utilized more creatively and with greater latitude, so planning can be done by going back in time even when things seem to demand an immediate decision, the light can slow time and bend it, in effect, to create ample opportunity for all of the prior discussions, planning, and agreements to be obtained and sorted through and agreed to and accepted by the parties who will be interacting with that individual planning to reincarnate quickly. Your perception of time being linear and unwavering is an illusion. It is not that time is not real, it is that time varies with respect to the vantage point of the observer and so it is relative. Just like the velocity of moving objects, it will vary depending on the vantage point of the observer because it is relative to their motion. So in the divine realm, time is simply not a problem because it can be navigated in a way to accomplish any planning needed.