This all sounds very reasonable in a vacuum, but I don't really see how it's not been falsified by now.
We also haven't really ever tried anything even close to a pure democracy. There have always been entire groups of people who haven't been allowed to vote, and all democracies have always had parties with more seats (and other measures of political power) than support and vice versa.
Sure but that's just on the margins. The problem in the USA right now isn't that Republicans have some systemic advantages due to the electoral college and senate map, it's that millions of people vote for Trump -- and if he had the same charm but was less politically stupid, that number would be a lot higher. The problem in Germany isn't {whatever systemic critique you could have}, it's that a ton of people are legitimately persuaded to vote for the AfD. The problem in Finland... I mean, I have no idea what the problem in Finland is.
Not that many people in the US are actually happy to be voting for Trump, they do it because they don't feel like they have better options, which is not that hard to happen when they only have one other option and most of the other option's voters also agree that they aren't happy to be voting for him, they just do it because they don't feel like they have better options. The fact that most Americans are in agreement that there aren't really any good options to vote for shows 1) that the average American does actually have reasonable takes on politics 2) that the American democracy is fundamentally extremely imperfect.
Correct me if I'm wrong, but it doesn't seem like AfD really has all that much power in Germany?
In Finland, the far-right Finns Party is currently in the government, but it's not really a far-right government, it's just a regular liberal conservative right wing government. The government is basically doing all of the things the Finns Party promised voters they wouldn't do and extremely few of the things that they promised they would. So it's not really a disaster, it's just a disappointment and the majority of Finnish people agree that it's a disappointment, only a minority actually support the government, so in a perfect democracy, we wouldn't have this government. The Finns Party presidential candidate didn't even make it to the 2nd round in the currently ongoing presidential election, where the 2nd round is between a centrist guy from the liberal conservative Coalition Party and a centrist guy from the Green Party (which is a green party but without the tankies and 50% less anti-nuclear sentiments than you would expect from a typical green party).