Agreed. But I hope my point makes sense. If 18 percent of people vote for someone and get no representation out of it they quickly will get disillusioned about voting and the country's politics will not be representative of the people.
Yes. Because by definition in a presidential election there can be only one winner. In the House or Senate there is no reason to not have proportional element.
Presidential elections can't be proportional, true, but they could be made less polarizing. The goal should be consensus, the election of a candidate acceptable to the vast majority of the population, not someone loved by 51% but loathed by the other 49%.
Those Clinton voters still have their representatives in the House (and probably in Senate, too), so they are represented in the political system, just not in the White House.