Let's say that in 2020 Corbyn is leader, he will get a lot Green support, (let's say over 50%) - that's just over 500,000 votes. However to win next time Labour need to take votes and more importantly seats off the Conservatives - and they need to take around 2 million of them. My response would be could Corbyn really persuade voters from the Tories to come across? As without it, he would get nowhere near power, they need to take 100 seats - most from the Conservatives and need a swing of over 9.5%.
Labour certainly stood on their most 'left' platform since 1992, also turnout was at its highest since 1997, its impossible to predict which way nonvoters would vote.
Um. In a FPTP voting system, you really only need worry about the marginal seats, so capture about 300,000 votes in 30 - 40 key marginals and Bob's your uncle (as the Tories demonstrated in May). In my constituency, the incumbent Tory MP got 50-odd % of the vote, with a 65% turnout, so 17+% of his vote was actually wasted, so total votes cast nationwide means absolutely nothing. In this scenario, Labour would need the support of the SNP (assuming a "hard left" Labour leader fails to woo the Scots back). That's why the voting system is so shit, as it enables this sort of "democracy".