ok my thoughts is to become a lingering spirit you have to be defeated by another keyblade wielder or lose your body to another keyblade wielder

and

if you destroy a heartless and a nobody then the original person comes back wouldnt that mean that we could never destroy xehanort because it would become a nobody and a heartless and then when there defeated the original person would come back

well i dont know but the lingering spirit (like vanitas) kinda looks like it could be the nobody of vanitas (since hes like all white) but like i said idk

I have no idea what you are talking about. What are you saying?

Every single Org.XIII Member has been defeated by a Keyblade Wielder, and you don't see them with a Lingering Spirit.

 

And yes, Xehanort could be destroyed...he can just be killed ._.

i mean if a keyblade wielder is defeated by another keyblade wielder

plus, they'll probably defeat him b4 he becomes a heartless/nobody