Heh...the Mirror Universe, and what DS9 did with it, is a whole other thread.
As to what humanity is in denial about--their paradise could NEVER have happened without a steep price being paid. Something tells me there was some serious revisionist history that went on. As for human arrogance--it's a combination of arrogance and naivete, really, and it is an incredibly dangerous one: to believe that they are the moral arbiters of the universe, yet be so incredibly naive as to how the universe really works, and what the human(oid) nature REALLY is that so often they are unwilling to take a stand...it's pretty sad. Now, DS9 went a long way towards changing this, thank goodness.
When it comes to the Cardassians, I DO think we saw evidence that the change was beginning--through the leadership of Damar and Garak, individuals who both had to change tremendously in that final arc. I believe their chances of true reform are greater because they had a crisis that absolutely shattered everything. Humanity may require a similar crisis--hopefully not, but it's likely. It doesn't have to be a physical one...even a severe scandal or PR crisis could do it.