I'm trying to communicate that mental/personality disorders don't work like that.

It's a very costly and stubborn mistake either way and he has (or hasn't) bipolar either way.

Even in the most severe substance dependence cases some people can override their short term wants (eg. they don't go and mug people to get money for their next fix), while there's a clear pattern of this behavior in others. (But it's what we expect, since we define one by the other.) So in both of those instances, it was their mental state, but was it the altered part or the underlying base state that let the dependence to form in the first part? It's really not separable.

One explains the other, but there's no direction, no proximate causal relationship. At best there is some predictive power, ie. if someone goes off their meds, or stressors increase they will show behavior consistent with this or that. (But in general everyone will make more mistakes when they have more stress in their life. Who would have guessed, I know.)

Yet these categories (diagnoses) are not useless. They communicate behavior patterns, things to look for, things to be mindful of and try to manage.