substance abuse is certainly correlated with an increase in mood disorders, although it's the "chicken or the egg" dilemna; it's difficult to say which comes first. That being said, a lot of people with bipolar disorder will abuse cocaine and alcohol to regulate their mood, as a form of self-medication. I would not be surprised if in the end we were to learn that in fact Williams was suffering from bipolar disorder... Which is a genetically, neurobiologically determined condition, for most of it, much more than unipolar depression.
I agree that addiction could have led to his psychological disorders. But i consider addiction to also be hereditary in some way. I've learned that an addict is literally born an addict. Its a gene inherited at birth. Same thing with bipolar disorder. Various hereditary factors can often (and usually) lead to more serious problems such as the ones listed.
p.s. I'm not a psychologist per say, but i have a university degree in Psychology. I've also discussed my theories with other psychologists and they agree with mine. Of course, i have no doubt other 'shrinks' as you put it would totally disagree with theories others agree with.