Does anyone have the dialogue that was cut out during development which takes place while Noctis was in the crystal in Chapter 13?
N : Noct
B : Bahamut
B: The King’s Light is yet faint.
B: When it shines the equal of the Stone, I, Bahamut, shall pledge my blade unto him.
B: Enter now into Reflection.
B: Gather unto the ring the Light that will purge the scourge of the stars.
B: When the time is come, the Usurper shall know defeat at the hands of kings true.
B: The Chosen shall embody the Stone, the mortal will...returned to naught.
N: Returned to naught? I just...fade away?
N: (My life for the power to save the world...)
N: I'll do it.
N: I'm not ready.
N: Wait, no! Lemme outta here!
B: The Light waxes full, O King.
B: Go forth to the throne and cast out the darkness, that the Stone's Light may reign.
My theory is that you saying "I'm not ready" was going to have Bahamut send you back to the "Forest of Memories" to look up memories of Luna, Regis and other things (presumably where the Luna cutscenes were before Tabata "didn't know what to do with them"), something that we saw in Platinum Demo. And this would also allow you to return to finish any past quests, but they decided to give you Umbra instead as they didn't include the Forest of Memories. The "special events" said to occur here may have been where they plotted to have any dream-related stuff, such as maybe Platinum Demo, Terra Wars and Moogle Chocobo Carnival.
The Platinum Demo is the world of his dreams, featuring a forest; it's likely this same forest area was the "Forest of Memories" that morphed into the "Astral Memory" Umbra lets you use.