re 2, you'll see at http://proselint.com/approach/ that one of the guiding principles of Proselint is that we defer to experts. In practice, that's meant almost all the advice comes from Bryan Garner's usage guide, Garner's Modern American Usage. He is a careful compiler of advice and you'll find that he is almost never "totally wrong", and when his advice is debated, he knows it, notes it, and provides a thoughtful discussion.
re 1, we think of Proselint as eventually being useful as a training tool, a way to learn the conventions. Note that natural languages are large, with so many low-frequency terms that nobody can learn the whole language. Why err if an automated tool can help? Consider for example demonyms, what you call people from a certain place. How many people know, for example, that people from Manchester are Mancunians, not Manchesterians? Rather than call someone by the wrong name, with Proselint the voice of an expert gently corrects you, and you learn a cool new word.
We aren't a mob of programmers, we are three people who love language, respect it, and think we're 2% of the way to making a great tool, one that The New Yorker could run over its stories to flag issues that its own editors would flag anyways. (In fact, we've done this, running Proselint over a corpus of highly vetted text, and have found numerous issues.)