Apparently there was talk among the PLP of forcing Ed out if Labour lost Heywood and Middleton in October of 2014, where they only scraped by just over 600 votes ahead of UKIP. What is more, we know that Ed would have gone had he been defeated in a no confidence vote.
With so little time remaining, they would likely look to install a Michael Howard figure to fight the next election, then Alan Johnson would be the favourite for that job, there are question marks over whether he would have taken it, but I wouldn't totally rule the possibility out as he has suggested that he
regretted not running in 2010. If no compromise candidate could be found, then the favourites in the subsequent leadership contest would likely be Cooper and Burnham, who were touted as the two main candidates for much of Milibands time at the helm. Balls wasn't all that popular with the public, and wouldn't run because of Cooper, and since this leadership election would be far more about choosing a leader and far less about debate, there is less of a possibility of a hard left candidate being brought onto ballot through sympathy nominations as in 2015. Burnham would likely win any leadership election, as he was the more left wing candidate, and would have the major union backing, and we now know that the Labour membership were more to the left than first thought back then.
So Miliband's successor would either be Johnson or Burnham, depending on how they were chosen. Though the public might frown on the instability, I think after the dust had settled on the relatively short electoral process, they would take a liking to either of them, and Labour would be boosted in the polls. In both cases, the hard left would be angry, with Johnson because of his rightward leanings and the lack of democracy in his appointment, and with Burnham because he would likely run on a platform that was slightly to the right of Ed (who most though had gone too far to the right already), as he initially did in OTL, but unlike in 2015, there would be no Corbyn to siphon votes off from him to his left.
Scotland was likely already gone by the end of the referendum, only a half dozen seats or so could be salvageable no matter who the leader is. I think Johnson would definitely do better than Miliband in OTL, though, probably ending up as the largest party in a hung parliament dependent on SNP support, which would be difficult to say the least. Burnham on the other hand would likely not withstand the exposure of a general election campaign very well, he would have floundered just as much as Ed on the SNP question, and we would likely have still gotten the mush of policies not adding up to a coherent vision. Maybe his novelty would net Labour a dozen or so more seats than OTL, but still just off of forming a government.
David Millibrand. Joke
Can someone tell me why they ran against each other for the Labor leadership? This doesn't happen often with brothers.
Cos David was on the right of the party whilst Ed was on the 'soft left', and he didnt believe his brother or any other candidate had the right ideas to change the country, or at least that is what Ed said. For all his flaws, he doesn't seem like a particularly dishonest guy, so I believe him on that.