I have problems with Stevens' coaching as well - the biggest concern being that he does not force his players to do the right things and stop doing the wrong things. He seems too worried about being a "player's coach" than anything else.
That said, the lead was blown because Irving refused to run the offense and just went total dribble-iso or dribble-penetrate. Yes, sometimes he will attempt a single pass out of the dribble to a teammate, but that is missing the point. The rest of this team is more effective within an offensive system, not standing around while Irving dribbles the ball. Regarding defense, the guy runs hot and cold - he was lazy defending the 3-point line vs Lakers.
As the season has progressed, Irving does not seem all that happy to be here and some of his teammates aren't all that thrilled about playing with the guy. Personally, I don't think Irving will ever be fully content wherever he goes, Knicks, Lakers, Nets, whatever. Just not in his nature.