After watching the ‘Show Yourself’ debate unfold - the irony of the situation is that the answer regarding who or what the voice is - still ended up being so ambiguous in the final cut.
My interpretation of the voice is that is has always been Elsa’s - her character arc throughout both movies has always been internally motivated.
With ‘Let it Go,’ Elsa gave herself permission to be herself and experiment with her powers. With ‘Into the Unknown,’ Elsa allows herself to be seduced and listen to her ‘inner-voice’ that is calling out to her internally. That the Queen of Arendelle is not where she is really meant to be - to take the risk of leaving her new comfort zone, and lose everything to follow her passion.
This all then culminates with ‘Show Yourself,’ where Elsa finally comes to the realization and full acceptance that she being herself was the answer she has so desperately searched for her entire life - closing and completing her arc throughout both movies.
I feel like the directors/producers/song writers finally all decided on the choice of making the voice Iduna as a simple and nice book end to the ‘All is Found’ lullaby within the context of the movie - though story wise, considering Iduna has no magic, has never expressed a direct acceptance of Elsa while she was alive in either movies, and that she and Agnar have long passed away before they ever got a definitive answer regarding the ‘source of Elsa’s powers’ - I feel like it only makes sense that rather than the voice literally being Iduna herself, the voice is more of metaphor or a figurative representation of Iduna to help Elsa come to the realization and confirmation that she is deeply connected to Ahtohallon, by reminding her of the lullaby that she was comforted by as a child.
Otherwise, if the voice is literally Iduna herself, I feel like it greatly diminishes Elsa’s arc by having the need for an external force in her mother to propel Elsa to follow her passion into the Enchanted Forest - as well as the need to have her mother’s validation and approval for Elsa to finally feel free to embrace and love herself.
I wish the docu-series would have explained more regarding what was so unsatisfying or confusing regarding Elsa seeing, singing, and directly interacting with her own reflection before transforming into her final and complete form - it certainly sounded like it would have been much easier to follow than the final version.
All this said - ‘Show Yourself’ is still incredible and it was 100% worth the time invested that was spent to save it in the movie. Just a shame it wasn’t the song Disney chose to nominate for an Oscar.