Did you read the interview of Nikhil Advani about KHNH? He himself told that Karan did almost all the works in KHNH. He said that "It was Karan's baby all the way". Karan never took it's credit. All the staffs gave the credit to him. Karan made the story of KHNH to direct it by himself. But when he felt that Nikhil, who was his Assistant Director should have a film to direct, then he gave it to Nikhil because the story was ready. He gave his baby to Nikhil just to give him a chance.
And as Karan always says- when he has the top actor of India in his hand, then why should he'll take other actors. Ask the people who knows Karan and love SRK. The one who ever loved sRK, can't even think of doing films without him. Not only karan, also Aditya Chopra, Aziz Mirza, Farah Khan. Karan's dad Yash Johar asked him before his death to always include SRK in each of his films. Because he loved SRK as his son. And about Preity, she is just a lucky gitl! Otherwise Karan always would do films with Kajol. They are not too proffessionals. They always give priority to personal realtions also. And why not, when these peoples are in the top chart of Bollywood?
I agree with you that Karan has a long way to go. He just started his journey. And that the same thing Karan told. He is getting matured about films after each time he is doing a new one.