For the seven-way ("dwarfs'") debate: I'd say that it hasn't, yet. Even if they 'empty chair' him, it'll depend on how the other leaders handle it. If they dive in with the jokes too fast, it'll all get too silly; they risk looking like little children squabbling while the important adults aren't even in the room. They'll need to lay on just the right amount of ridicule, and that'll be difficult to do as they try to one-up each other.
The best thing Ed could do in such a case would be to avoid squabbling. He needs to look statesmanlike, so people look at the six politicians and one empty chair and think "Yeah, that Ed guy, he's the natural leader of the bunch".
And as for the one-on-one:
the BBC, ITV, Channel 4 and Sky have said they will continue with three debates in April as planned. They have made it clear all the debates will happen regardless of whether invited parties turn up or not - and today described the head-to-head debate, with Mr Cameron and and Miliband, as "vital". ... And to complicate matters further the one-on-one contest would have to comply with the broadcasters' own rules on fairness and balance - perhaps the basis of a challenge from Number 10.
It's absurd to suggest that a one-on-one debate between the Prime Minister and the Leader of the Opposition can go ahead without both of them; it'd be nothing more than an hour of air time for the other, with no rebuttal. Some may think the Prime Minister deserves that as some kind of a punishment for not agreeing to something he has no constitutional obligation to agree to, but in any case, there's no way that could get past the rules on fairness and balance.
It's posturing by everyone at this stage. Presumably, the final deal will be reached behind closed doors. Some apparent concession will be offered on the orientation of the water bottles or the hairdo of the presenter, some minor change, the PM claims a moral victory and agrees, having made it clear he doesn't need these debates at all, but has deigned to take part, while Ed has been practically jumping up and down all the time because his personal ratings can only improve and everyone knows it.