I suspect that the powers behind the Unite union have long had the opinion that Macdonald and Corbyn are good enough actors to hoodwink the electorate into believing that another Labour government would be far more cuddly than yellow streak Brown's effort, which was rejected 7 years ago.
Ideally, what the power-brokers would like is someone with yellow streak Brown's intentions and Bliar's bonhomie, but there isn't anyone like that. Second choice, I think, would be a dupe like Bliar, who'd give the Stalinist / Markist a free rein in return for personal aggrandisement. I'm not sure they have anyone well-enough established to fill this role. So they are left, I fear, with the third option which is to choose a Stalinist/Markist and then work overtime to convince the public that he's really a social democrat - with the whole thing made more difficult by the fact that none of the contenders is enough of a chameleon to be able actively to do much to help this process.
But the challenge of persuading the public that Macdonald and Corbyn -led Labour is a social democratic party must have a degree of difficulty not far short of trying to do the same thing with a party led by Arthur Scargill. Or even one which had concluded that its best interests lay in reappointing the Great Helmsman of Kirkcaldy himself.