Keith:
In order to have a really good discussion, I think you are going to need to define pessimism and optimism. As you left it here, I think your starting points were too vague to generate a good conversation.
That said, I'm going to take a stab at addressing one of the issues you raised:
1. "Why is it that cynicism and unhappiness represent movement towards maturity?"
This, to me, was the most interesting question you asked. I think cynicsm and unhappines most often represent movement towards maturity because they arise out of experiences over time. Thus, as we grow old, we learn that our innocent ideals and dreams rarely are instantiated in the events of life. Experience with the world shows us that things are much more difficult, less ideal, and uglier than we had hoped. Now, even with this sort of information, people still have a choice in how they respond to it. They could become cynical, disillusioned, and unhappy since life is not as they had hoped or expected. Or they could become doubly motivated to pursue their dreams and realize their ideals, against all odds. Or they could say that things really aren't as bad as they seem and try and view their experiences in a more positive light (change their view rather than their circumstances).
I think the first response, the cynicism and unhappiness, is viewed as the most mature response since it easily masquerades as the only response that is serious, sobering, intellectual, and arising from deep thought. It also strikes a chord with emotions such as anger, hurt, resentment--all sorts of inner pain that, if felt, are usually felt strongly and in great measure, and thus are often perceived as being more real. Why is this? When we stop and seriously consider our lives, we must face the fact that they are not as good, happy, or ideal as we had hoped. We are forced to confront the evil within us and the evil without and around us. Evil and the pain that is associated with it, often is so strong that it seems to be the most real thing in our lives, perhaps even the only real thing as its shadow stretches back over even our earliest memories. When we think of "peppy, fun, 'Hey, let's play a banjo'" types of people, we tend to think of them as being frivolous, unreflective, and not very real. We may find ourselves thinking, "They must have never experienced the real things of life (i.e. the painful things of life), and that's why they continue in their innocent merriment."
Now, why is it that pain and evil seem so real, while mirth, joy, and happiness do not? I think one reason is that we have not learned how to be happy. We are so obsessed with our pain and hurts that our attempts at joy and happiness are really just attempts at forgetting our pain. We go out and have fun, not for its own sake or for the sheer joyfullness of joy, but as a means of forgetting our pain. We attempt to use joy as a narcotic, to dull the pain, forget the hurt, and pretend that all is as it should be. Thus, when our merry-making comes to a close and we are sober once again, we find that the memories remain, the pain remains, and the narcotic of mirth had only a temporary (and not real) effect. After a while cynicism becomes our general response to the pain and evil, and this is viewed as prgoress and maturing from our childish attempts to escape the pain with desperate attempts at merriment.