I'm very new here and I don't upvoted my comments. In my mind comments are different from posts. Comments are more like a thing I want to tell the author whereas posts are different altogether. I do upvote my posts, which is a habit I took on from youtube. The reason is exactly the same as yours, just to show my viewers that I like/approve of my own videos. Although like your research shows, the actual effect over here is quite negligible, especially considering my very low SP. It's mainly a mindset thing.

Again, thanks for a detailed post!