But who's to say "my read" was any more correct than theirs?

You are. If you don't have conviction in your positions, nobody will. It is my view also that Hachiman speaks some of the truth, but leaves a lot out. He is unhappy and unfulfilled because his approach to life is fundamentally wrong, but his somewhat accurate observations signal to him that he is on the "right path".

On adaptations, an adaptation must adapt. Lord of the Rings was a great adaptation of the book because it was adapted to the screen. This meant that Tom Bombadil was removed from the story, despite being very important in the book. The reason for it was that Tom Bombadil simply did not work in a movie.

To contrast this, the Watchmen movie tried to show the comic in movie form but did not adapt it into a movie. They used the comic as a script and the movie turned out poorly.

A recent adaptation is Devilman: Crybaby. In my review of it I mentioned how it was better than the original. The faithfulness of an adaptation and the quality of an adaptation are separated. The only reason that "the books are always better" is because the books usually are better. The changes made to them in botched movie adaptations make the end product worse than the original. Devilman did not have this problem because it became a Yuasa story, not just an adapted Go Nagai story.