No No the theory of Roxas having the heart of ven is impossible because as you can see or read online Roxas is Sora's NOBODY and he is able to wander through the darkness like Diz said in KHII and Ven's hearts is made of pure light when xehanort extracted the darkness. Actually Sora and Ven's are still MELDED because it was still in repair and you can't take a hearts that is melded apart. I suggest if you came up with roxas has a heart theory and support it replay all the kingdom hearts game and you will actually learn