IFS can be an amazing therapy. It helped my fiance immensely. He struggles with delusions and dissociation, not to mention depression and anxiety. But IFS helped his mental health more than any other form of therapy ever has.

The idea of the mind being multiple can't really be proven scientifically as it's more a lens through which to look at the self. That is, the mind is completely subjective depending on the person. So some people resonate with the "many personalities" paradigm and some do not. But you can't prove it just like you can't prove any qualia. All we can do is see what's helpful.