The postmortem was grim. In the room, leading Labour MPs, recent party staffers and assorted Labour figures of long experience pored over the entrails exposed by the high priests of polling and academia. Pointers for the future lie in this gruesome raking over of the details. Too many in Labour give only token nods to the cataclysmic abyss that has opened up between the party and the voters out there.

To win, Labour needs another 124 seats, or 60% more MPs – an increase not seen from any party since 1945. According to Greg Cook, the former Labour head of political strategy, among those 124 hypothetically “most winnable” seats are an improbable 16 from the SNP and two from Plaid Cymru. Failing that, if all gains must come from Tories, Labour must seize North East Somerset, held by Jacob Rees-Mogg with more than 50% of the vote. Gerrymandering Tory plans for boundary changes will take at least 1% off Labour’s vote.

Between now and 4 April the Unite and Momentum machines will grind exceedingly hard to keep their grip on the party

Unprecedented doesn’t mean impossible, never say die: despair after the great 1992 election shock left many reading Labour’s funeral rites. Yet that same year Black Wednesday struck the Tories down as the UK fell out of the European exchange rate mechanism, and within no time, John Smith, then Tony Blair, applied the defibrillators that jump-started Labour back to life. But the task now is of another order: like climbing Everest backwards, blindfolded and without oxygen.

This meeting of party figures was not a cabal for a leadership candidate, but had supporters of four contenders (held just before Jess Phillips dropped out). None, I think, were Rebecca Long-Bailey backers, with her daunting Unite and Momentum support. That’s the great schism, between the Corbynites and the rest, that no candidate dare name. It’s a split each candidate claims to be the one to mend without quite defining.

This deep-dive exposed just how appallingly the election was run at the top, a hair-raising picture with graphic examples; worse than anything I’d heard so far. It became a kind of therapy for those who had endured it. It highlighted abuse from the doorstep by former Labour voters who feared Jeremy Corbyn, called him unpatriotic, antisemitic, spendthrift, untrustworthy on crime, and a lot more.

But worse was their own party’s bullying, favouritism and fixed seat-selections; the sudden removal from Bassetlaw of a non-Corbynite candidate was a prime case. Fixing was not unknown under previous Labour regimes but this was almost beyond belief. Only true Corbyn candidates were rewarded with volunteers and resources, regardless of polling showing where help was most needed. Some seats were needlessly lost as a result.

Some party officers, chosen for their loyalty to Corbyn, had never run any election and failed to get out the free postal forms until after the postal ballots had arrived. Shadow cabinet ministers tell of arriving at TV studios to be asked about Labour’s latest policy of which they knew nothing – sent out cluelessly to defend the £58bn for Waspi women, or free broadband.

Why pick over the bones of the election? Because places on the National Executive Committee, which oversees the party – and which is currently majority-Corbynite – come up for election with the leadership ballot. Will the membership yet again opt for Momentum-slate candidates who vote with craven obedience, or will they choose people who think freely?

Long-Bailey on the NEC has loyally voted with the slate, failing to deal with the antisemitism cases that darkened Labour’s name. She agreed to the attempt to abolish Tom Watson’s deputy leader position. Jon Lansman, the Momentum leader who led this failed drive-by shooting, now runs her campaign. Awarding Corbyn “10 out of 10” for leadership, Long-Bailey calls for even tougher mandatory reselections of all MPs, though that split local parties ahead of the election.

Len McCluskey vigorously promoting Long-Bailey on Monday’s Today programme, ascribes election defeat entirely to Labour’s failure to back Brexit, blaming Keir Starmer. Others see Starmer as the man who prevented a massive flight of remainers – and the abandonment of the party’s own principles. Had Labour sided with Tory Brexiteers, what might they say when those northern seats suffer Brexit blowback?

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Labour MPs’ disastrously ill-judged attempt to remove Corbyn in 2016, after his reluctant stance in the referendum campaign, created a rift with the membership. But that may be closing, as the party dangles on the edge.

Affection for Corbyn remains strong: no wonder no candidate dares say honestly what went wrong. Nor dare they pledge to clear out Corbyn’s cronies from top party posts: shockingly the Observer revealed a series of key party positions are being rapidly filled with Corbynists to tie the hands of a new leader. Yet, on the ground, energetic young Momentum and other members have been nose to nose with voters who told them briskly why they weren’t voting Labour.

Labour splits many ways. Oddly, its centre-left Progress wing, which backed Jess Phillips– likeable but not a plausible leader – turned against Starmer for staying in Corbyn’s cabinet and failing to denounced him now. They pose as “realists” ready to do whatever electability demands, yet fail to engage with what getting chosen by Labour’s membership requires. True, Starmer’s no centrist but from the soft left, aiming to make the best Labour policies credible, while the hard left call him Tory-lite and worse.

Yet unity may be easier than it now looks. A convincing Starmer victory would bind up wounds; some Corbynites might return to the socialist splinter groups from whence they came. Most will stay – and people change when times change, as they did in the 1990s.

The result hangs in the balance. In polls Starmer is ahead of Long-Bailey, with Lisa Nandy’s intelligent campaign gaining ground, though few of the 650 constituencies are yet revealed. The signs are that most election-shocked members sincerely want someone to win for them. But between now and 4 April, when the result is declared, the Unite and Momentum machines will grind hard to keep their grip on the party. If they succeed, a party well used to roller-coaster rides faces its greatest existential crisis since the 1930s.

• Polly Toynbee is a Guardian columnist

• This article was amended on 29 January 2020. An earlier version stated that no party had ever gained an extra 60% of MPs in an election; in fact this happened in 1945 and in other previous elections.