I think giving Pakistan Azad Kashmir and the Kashmir Valley, Giving China the Shaksagam Valley, and Giving India the rest is fair. That way, Pakistan and China get some stuff, while India gets all the IMPORTANT mountainous terrain, as well as K2.
More seriously, could you add an option that's just "All"? Because I feel like it skews the numbers when people think it belongs to several countries and thus puts down all the countries. Like right now, the numbers in the results add up to more than 100%, so it's not really accurate picture of what people think.