FWIW every time I have ever flagged something on HN it’s been a fat finger mistake on mobile. Probably a drop in the bucket but worth considering that the signal is still polluted even with perfectly good faith user behavior.
[-] collapses the parent, you're correct. You can also unvote if you change your mind on your vote.
When logged in, I have upvote buttons on all comments, and I believe downvote buttons are enabled at over a certain amount of karma.