Watching my mother-in-law decline through Alzheimer's made me question everything I'd ever thought about God and the human soul. Who is this person who behaves so differently from the person she once was, and retains so little of her former life? If there is a soul that's separate from the body (and the physical brain) why isn't it allowed to express itself at the end of life? Is it just eclipsed by faulty wiring, but ready to emerge restored into the afterlife at her passing?

This is indeed very frustrating.

And if there is an "Immortal Soul" that can learn and grow throughout life, but can also decline and suffer, just which version of the thing is going to get scanned and uploaded into eternity?

Beautiful questions you've got here. Don't stop searching. I believe as many say: The body dies and the soul goes. Just as in a computer; The physical body (hardware) remains but digital information (software) travels throughout the vast "internet" eternally.

If consciousness evolves from the unpredictable behavior of complex systems, I think we do it a disservice by reducing it through religious narrative simplification.

... still trying to wrap my head around that "consciousness". The word itself feels like it has so many meanings... notice the new A.I robot that came out named "sophia".

But on the other hand, how do people go on without such a meaningful narrative to sustain them?

They most likely follow the "good intent " rather than its content.