Hmmm, think neither schizophrenia nor this extreme case of split personality is a good scenario...
There's stress induced delusions as an option too, it's less severe then schizophrenia although it's roughly related, big difference however is schizofrenia is more common and usually surfaces from puberty to mid twenties while delusional disorder is rare but often occurs midlife... It's likely Bella would have been diagnosed much earlier if it was schizofrenia.
And then there's a possibility it's caused by intoxination too, alcohol, drugs but also wrong medication and such.
Ah well, loads of possibilities anyway... so i hope there's a better diagnoses then your best case scenario.
(ah, and check out "hide and seek" if you didn't already!)