As for pointing out queries to authors I am in two minds. Many stories I read are actually several years old and, with the passage of time and their continuing writing, I do wonder whether most authors can be bothered.
You get all types of authors. While many are eager to correct any and all errors, regardless of how old the story is, others simply don't want to be bothered, considering stories finished and done once they post them. The lesson here is: you can't draw conclusions based on how one or two respond, as each author is different.
Generally, it's bad news to start with a list of 30 errors, as it's intimidating, and it comes across as bitching about the quality of the story. Typically, I'll start by telling the author why I appreciate their work, then conclude by one or two typos I noticed. If they respond positively, I'll send a few more, noting whether they actually change anything or not. (They're not obligated to, but how they respond tells you how they deal with suggestions. I'll normally tell helpful readers how I resolve issues raised.)
Generally, authors won't ask for help until they've gotten several messages from you, and your advice is routinely helpful. If so, then they'll very often ask for more. But again, not everyone does that. One clue, I specifically request corrections in my 'End of Story' notes.