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?
"I tell you what you all need, you need to take a thirteenth step, down off your high horse." - Hank Hill, King of the Hill