I'd would like to see the "join session" option for recent players. Sometimes I have a good game or two with someone, and I might get disconnected or the lobby might close. I'd like to be able to join up with them without having to add them to my friends list, (and thus remove someone else).
The thing is, that works both ways. Your party might get ruined by some kid joining in, and you'd have to make your profile private to avoid it.
I'm quite sure you can't. It won't show if you're looking through your friends list and someone is in a full game, but it never shows from your recent players. Possible, but I've played plenty of multiplayer games that allow this sort of thing, and abuse of it is fairly uncommon. And, like you also mentioned, it's plenty easy to counter.
