Can character be changed? Can there be a return to good character if one fails Jesus? If so, how? As we look to our future, we know that our leaders will fail us somehow. So should we trust, and if so, what in?
According to the Bible, the only quality of being "good" is accepting and being in a relationship with God through Jesus Christ. All the other stuff you mentioned are nice, but they don't constitute somebody who's "good" in God's eyes, and should thus be judged about the same as those who don't have those characteristics.
The parable of the Prodigal Son tells us that even when we fall away, even far and to the point of basically giving God the middle finger, He's awaiting our return, and when we do, He'll welcome us with open arms, even reinstating us to full-blown heirs.
The Bible tells us that King David was "a man after God's own heart". He was a great leader, but he was also human, and humans fail. Our leaders today: though many are liars, even those who aren't will fail us some time during their time in power, just as David did his people. I think that looking at the story of Daniel might help in answering your question: Daniel supported his leaders and prayed for them, the kingdom, and himself in serving them. But He believed in and trusted God. Also in the history of Daniel, his devout loyalty to the Lord influenced many of the kings/leaders and their decisions in leadership.