What's the difference between "Price Today" and "Overall Price" when you hover over an item? They seem to always be the same value.

none. It's misinformation.

Did it used to work?

It seems like it would be very easy to remove if it's no longer supported, so I guess I was a bit confused by that.

yes it did. Over the years got reduced from two month delay average to one month to two weeks and nowadays it's within a few minutes. Users used to complain all the time about the time delay being "too long!!!eleventy". Now they don't; and it's useless.

You are not wrong. It's fairly regular that in Suggestions someone asks for another item to be added into the hover and this suggestion no matter how good gets shot down by staff because there's no room in the hoverbox. If there isn't the room then this line isn't helping anyone and they should eliminate it. I have no idea why they don't.