Re: Why Khan?
They ruined this by going too far with Sarek. "I married your mother because I loved her." and "Do not try to." He can be angry? That's a can of worms.
There are times as a parent that you simply have to be brutally honest with your kids. I saw Sarek being brutally honest with Spock twice in the film. As a parent I appreciated that Sarek shared with his son that the world they inhabit isn't perfect.
Asa far as Khan goes, why not?
"Just give me two seconds, alright, you mad bastard!" - Montgomery Scott, Star Trek Into Darkness