It might be better to move away from philosophizing over the existence of mental illness and be more pragmatic about it. Vagueness of criteria and other considerations can be debated endlessly, so it may be better to view this in a different way.

In some people who have it, untreated depression progresses to where it's terminal ... i.e. they kill themselves. That this happens is not in question.

The idea that depression is not a pathology, flies in the face of this; it's ridiculous to assume that it is somehow "normal" to be so morose and melancholy as to kill oneself. Another way of putting it: Every suicide that occurs is a tangible demonstration that depression is a pathology and therefore that mental illness is real and needs to be treated.

Any philosophizing which swerves out of the way of this fact, is therefore useless.