The font-color of the comment (aside from links already visited) should be enough to indicate if it has positive or negative karma. Individuals can decide whether to upvote or downvote further at that point.
I'm not sure it's still happening. I think it stopped the people who provide corrective upvotes.
This is by far the biggest problem I have with HN. If there's not enough light in the room, I have problems differentiating between shades of grey/black if the contrast isn't high enough. And it makes downvoted-to-hell comments extremely hard to read because all the mobile apps seem to replicate this, and you can't just ctrl+a to add contrast.
Yes, this is true. However, consider a comment with 0 points. If two HN visitors see it at roughly the same time, and both upvote it, it will have 2 points, rather than one point. What's more, neither visitor will see the effects of their action until at least one page reload and some time has passed.