You have a point but nobodies don't always look like their somebodies.EX: Kairy-Namine
Also, xehanort transfered his heart but lost his body.
Furthermore, the Isa theory may be solid but it has a major problem. If isa has been controlled since BBS then he wouldn't have turned into a nobody.
Finally, Isa has to be a grownup since Axel(Lea) is.