1, They usually irritate me, I find them annoying and unnecessary in most of the stories. In a few cases it can make sense, but only in small doses and if it is actually some relevant and important information about the story. (For example: when the story takes a huge plot twist or there are different endings and the author feels this needs some explanation. I also remember reading a somewhat real life story where the author explained a few things in advance and I thought it was fine).
2, I think they want to feel connected to their readers this way and they also hope they will get more readers if they promise the story will get better. I don’t know how they succeed though. The other day I found a story, which started with the author apologizing for at least a minute straight then the actual story was another 1 minute and I was like “Okay, what?”
3, Honestly I would say only at the very end of the story… Like “Thank you for reading blah blah blah”. But as I have mentioned in the first point I can imagine a very few cases where I find them OK.
4, What should be included: if the story has very mature themes, contains really distracting scenes, abuse etc., I’m always glad to know this at the very beginning because I don’t want to read anything like this.
Things that shouldn’t be included in my opinion:
-what the story is about (We’ll see it, right?)
-why the episodes are short
-why the story has plotholes
-why the story has directing errors
-that it will get better and longer
-how many lines are in a certain episode (Why are they including this? Most of the readers don’t even know what this means…)
-many other things I don’t remember
5, I don’t consider them author’s notes. I mean if the writer is asked the same question over and over again I understand he/she doesn’t want to reply to every single one and I think it’s nice to include FAQ in this case.
6, Ahh, the dancing/kiss blowing/winking author characters! Please no… I mean if someone manages to do this in a funny way, that’s cool, but usually that’s not the case.