I really don't think mental illness can be an evolutionary advantage in the long run. It seems to me that in pretty well all the 'developed' nations of the world, most of what ails us at so many levels is isolation. Families are isolated from each other, generations are isolated from each other, and so so many individuals never really form healthy nurturing relationships with anyone. And how we are supposed to feel good about ourselves mostly has nothing to do with our 'selves' any more, it is all about how we look, how much we earn, what we own, maybe who we know, etc
You are also right about the expectations thing. Another way to look at that is from the perspective of Maslow's hierarchy of needs. People who are fighting for food and shelter don't have time or energy or anything else to spend on trying to satisfy emotional or self-actualization needs. Ironically, those of us who are fortunate enough to have the time to pursue that level of things live in a society that does not validate those needs or provide any support for their fulfilment. (Yes, I am generalizing ere, there are pockets of sanity here and there!) Even the professions that originated in wanting to help people on those levels are being co-opted more and more by the quick fixers. In a capitalist world, people need to be functional, not happy or whole.
I guess it is true that a certain amount of what I would call alienation can be attributed to seeing the world from a large enough perspective that most of what goes on makes no sense and one seems to have no place in it. If that spurs one to take action to improve even small things, then that could be an advantage. But as someone who has struggled with mental illness for 35 years, I must say I really have a hard time with anyone wanting to put a positive spin on it.

Ok time to stop..geez who got me started on this