The theory is fascinating, but the Ienzo evidence doesn’t hold weight when considering the original JPN text.
本体の方とは初めましてだったかな
”I guess this was my first time meeting the main body.”
Vanitas having Ven’s old memories: is in the Organization with Lauriam and Elrena, canonically states he has memories from the past in the KH3 novel and that they could possibly be Ven’s
Sora having Kairi’s old memories: restores Radiant Garden, makes the difference in saving Aqua, forms a X-Blade with Kairi, Kairi cannot find memories past a certain point suggesting they went SOMEWHERE, Xion acts like Sora but looks like Kairi, Riku is shown working out of Radiant Garden during the search for Sora while he seems unsure Kairi will find anything, all the imagery of Sora falling from the sky or being sent to Destiny Islands