Pessimism? Depending on the definition that you give to this your answer might be different ... Pessimism infers negativity of some degree. If there is "pessimism" in Rose's message (and I think there is not!), it could be seen in the uncertainty that his description of the jury room process involves - it is not perfect and open to prejudiced manipulation, poor governance and apathy, but concurrently it is also open to a positive dissent that evokes the values that any civilized society governed by democratic processes and laws strives to achieve - a balance between individual and societal justice that allows for difference and for doubt, where the pragmatist might desire expedience and a retributive justice (revenge) as against a restorative justice dependent on a burden of proof provided by the prosecution and the presumption of innocence of the accused.
You also need to know what optimism means, to understand why Rose's view is much more positive. He has a hope for the democratic processes that take place in the jury room, even although they are flawed, they are still imbued with the liberty to speak one's own mind and allow for changes in opinion and attitude. Injustice has been avoided, although justice may well still have room to be achieved - doubt is allowable, indeed encouraged in a democracy. The jury room began with no doubt, became open to change and eventually achieved unanimity based on reasonable doubt despite continued division.
Fallibility? Again, depending on your definition of "fallibility", mistakes are inevitable; we are all flawed and open to error. Indeed this capacity to learn from error makes us what we are ... it also gives us the potential to identify partiality from impartiality, tolerance from intolerance, justice from injustice, etc. Rose highlights our fallibility (our very human flaws) through portrayals of individual stereotypical