1) You can consider the "Histories" as a read-only listing. You could make it into a playlist and then edit there.
2) It's something that had been requested, but probably not included (yet) since CDJs and XDJs don't have internal clocks and don't / can't keep track of the play time. rekordbox dj could do this, which is why it's on the feature request list.
3) Because you can search your collection.
4) I'm not sure ... there is enough room, but typically I'm matching by genre as well as BPM / key, so I'd never really thought about it. We'll pass that along.