Okay, the leadership situation in the Weimar Republic was weird. The man who appointed Hitler Chancellor, Paul von Hindenburg, was the President of Germany. The President was elected in a separate election by direct majority vote, and occupied a position roughly comparable to the monarchs of parliamentary monarchies. Like a monarch, his duties included formally appointing the Chancellor, equivalent to a Prime Minister. But, again like a monarch, the expectation seems to have been that his appointment would be a formality in recognition of whoever won the parliamentary elections and successfully formed a government. (Even today, formally speaking, Teresa May is Prime Minister of Great Britain because she was appointed to that office by Queen Elizabeth II -- it's just that the Queen always appoints the leader of the winning party.) Now, the Weimar Republic was young and had a very unstable political situation, and Hindenburg seems to have been deeply ambivalent about Hitler on a personal level, so there was some question as to whether he would appoint the Nazi Chancellor or actually use his discretion. But in the end, apparently out of a desire to mitigate the instability, he did appoint Hitler, to the world's sorrow.appointed Chancellor by the man that actually won the election with 53% of the vote in the second round of voting, and then Hitler snuck power to himself and the Nazis through parliamentary acts rather than gaining a majority.
We technically didn't elect Donald Trump, we elected the people that elected him, and he required a majority of the Electoral College to be declared the victor.parliamentarian selection of the chancellor works beyond secret ballot, so citizens technically didn't elect Angela Merkel, they elected the people that elected her, and I think that even then that requires a majority of Parliament in to be declared the victor.
Furthermore, had Trump not gotten a majority of the Electoral College, the Presidency would have been decided in the House of Representatives, where he would again have needed a majority to be declared the victor (which he would have gotten).
But you're right, the citizens didn't directly elect Merkel. They elected representatives from various parties who negotiated to create a majority coalition in Parliament. This coalition then elected Merkel. That doesn't mean as much as you seem to think, though. Like the monarch in a parliamentary monarchy, the coalition is always going to vote into the Chancellorship the leader of the plurality party. It's how that system works in practice. And the bare fact remains that of the ballots cast, only 35% were for Merkel's CDU/CSU party, which makes Trump's 46% look pretty good by comparison.
Again: the United States does not and has not ever selected the President based on the plurality of the vote. If no candidate gets an absolute majority of the Electoral College, we go to the House.
This just means the primaries are really important for determining the direction of the party. Which this election cycle ought to have amply demonstrated.
What's different if you're a single issue voter in a multiparty system? Say you vote religiously for the Pro-Life Party. Well, they're not going to get an absolute majority in parliament, so they're gonna be forming a coalition with somebody. And politics being what they are they're probably only going to be forming coalitions with parties on the right. So a vote for the Pro-Life Party is essentially a vote for a right-wing coalition that has "bundled" laissez-faire economics, nationalism, social conservatism, and other issues. Now, the relative strengths of those issues may wax and wane with the fortunes of the parties pushing them, but the same thing happens in the Republican primaries as well. This time around, for instance, the GOP decided it wasn't too interested in social conservatism or laissez-faire economics.
Sometimes parties refuse to compromise too.
In a multiparty system, you can cast your vote for a party and then see that party form a coalition with the side you did not expect. It's rare in practice, but it's possible. Imagine voting for a Green candidate and then watching the Greens join Trump's coalition. Very different situation than you yourself deciding to vote for Trump, isn't it?
And in a multiparty system, Johnson and McMullin get more votes, but Trump still probably wins the plurality because he's a charismatic demagogue who can attract a lot of votes, and then forms a majority coalition with Johnson and McMullin. Yes, there are a lot of factors that contributed to Trump's win. You have yet to demonstrate how the first-past-the-post system was one of them in any predictable or systematic way.
Just because some people defected to the right does not demonstrate that calcification is not taking place. Combined with everything else, it means that the Democrats are a weaker party than they thought. Even then, a strong Clinton cry in 2016 was it's Clinton or split the vote to Donald Trump. That's not a policy battle, that's weaponizing partisanship (which by the way in our current system is rewarding politics). By contrast, Donald Trump shored up his base despite being a terrible candidate with so much more baggage than Hillary Clinton. He had three 3rd Party challengers in his orbit, two of which that got mass media attention and more political experience (the third by the way was Constitution Party... which... yeah...but McMullin was probably a better candidate than Trump or Gary Johnson combined), yet both 3rd Party candidates support collapsed because putting Hillary Clinton in the White House was more terrifying than voting for everything that had been revealed about Trump (also, Gary Johnson demonstrated such ineptitude that he invented a new gaffe). So... yeah, it's blurry around the edges, but partisanship is what held Donald Trump together when Hillary Clinton didn't notice how wounded was in her final weeks of her candidacy, not Donald Trump's personality. I'd say that Donald Trump victory demonstrates calcification quite well.