It would be difficult for him to do it now after that little show.
No it won't - it'll be supply and confidence. All that was established there was what we know - Miliband won't go into coalition as he won't have a referendum or Trident on the negotiation table, but he will form a minority government knowing full well SNP will vote for anything they do, as not doing so would let the Tories in and be political suicide for the SNP.
There's only one outcome in this election as it stands - supply and demand, Labour/SNP. The only way that changes is if Cameron gets more seats than Labour and goes for a minority; but the moment he goes to pass a budget or whatever, that government crumbles.