The game's "cosmology" defines lack of existence to be the lack of Identity in that sense. Namine is described to be so unique and mind-buggling because even as a Nobody, she lacks what all other Nobodies have - a vessel. They all "become" Nobodies from the vessel, otherwise you can't explain Dusks and Creepers, or even the minor physical changes made to Nobodies from their Somebodies.
Saying Namine, who is in posession of a vessel, naturally not in its original form yet a vessel nonetheless - is nonexistent, is contradictory.
I wasn't mocking whatever you were trying to theorize about Roxas, mostly because I didn't really pay attention to that. All I was saying is that if you wanted Sora's B+S "out of the way", you could do so without saying it went to Namine, only with her still being nonexistent. As Namine being made of Sora's B+S, as they /became/ her - means she is still in a possession of a vessel and hence what's the point. And again the consequences not being brought to light makes me call shenanigans on this as this is no doubt something AtW would've mentioned.
If you wanna theorize about Roxas not being Sora's B+S, say Kairi pulled the vessel back when she purified Sora. Leaves you with Roxas without the B+S, and Kairi truly restoring Sora instead of doing something she was never truly implied to being capable of doing. She simply undid the process while leaving by-products floating around.
Makes more sense, as much sense as this thing makes.