Side question: does anyone know of a browser extension or anything that would hide/gray out HN headlines that I've already seen on previous visits so I know only what stories are actually new on the front page? Would make scanning quicker.

Accessing HN with Firefox (if your web history is turned on) does exactly that[1]. No extension is necessary. The same is true for every other browser since the CSS file it clearly says:

    a:link    { color:#000000; text-decoration:none; }
    a:visited { color:#828282; text-decoration:none; }

Sorry, didn't mean just visited links. I want unvisited (but seen) links to be treated differently.

Example: I visit once, scan 30 links, click 2, and leave. Come back 2 hours later, there are 5 new links, 5 have dropped off, and my attention is drawn to just the five new links, regardless of whether I visited the other 25 or not.

For links: your favourite RSS reader.