One thing that's completely missing from all discussions of Corbyn and Labour - even positive coverage of Corbyn! - is that one big reason Labour is behind is their complete collapse in Scotland in 2015.
In 2010, Labour had 41 seats in Scotland, the Lib Dems had 11, the SNP had 6, and Tories had 1.
In 2015, the SNP won 56 seats, 1 Labour, 1 Lib Dem, 1 Tory.
Corbyn didn't do that. Miliband (and circumstances) did.
If Labour could count on 40 more seats from Scotland, matching 2010, then...Yes I know there's a weird presumption that a Labour-SNP coalition is possible but nobody actually talks about it because, well, can't really win votes in England campaigning with the Scottish NATIONAL party.