This is commonly overlooked as it is difficult to notice, but note that Hashirama's buddha also has the 5 mini heads above its real head
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This leads me to believe that Hashirama's could also use all 5 nature transformations. I HIGHLY doubt that whoever is in Spiral Zetsu has mastered all 5 elements because there are only a handful of known shinobi who have done that. I believe that the 5 element attack is just an inclusion of the Shinsuusenju technique.
Also, as far as your point of Spiral Zetsu being someone who has sage mode or a curse mark, did you forget about this?
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The person inside clearly recognizes the Orochimaru's cursed seal and knows what it is which leads me to believe that this person is not in possession of it already (or any type of sage mode for that matter.) And since they are familiar with Orochimaru's techniques, I would imagine that they are acquainted already. Given all this, my guess is still Yamato.