Red lines grow redder and harder by the day on all sides in these intransigent ¡No pasarán! Brexit negotiations. The subtlety, sensitivity and common sense that eventually resolved one of the most intractable of age-old national conflicts with the Good Friday agreement is entirely absent here. Whatever little goodwill existed at the start has blown away on all sides, with Michel Barnier’s brutally unvarnished treaty terms on Wednesday eliciting explosive responses from ever more frantic Brexiteers.

Two of the architects of that masterclass in patience and pragmatism in Ireland, John Major and Tony Blair, vent their incredulity that peace after 30 years of terror and over 3,000 deaths is, in Blair’s words on this morning’s Today programme, “to be sacrificed on the altar of Brexit”. These two remind the forgetful what a phenomenal success it was to persuade republicans on both sides of the border to forgo their ancient claim to the six counties, unless the people of Northern Ireland should themselves choose a united island.

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In exchange, there has been 20 years of peaceful cross-border cooperation on everything. But now the Brexiteers say all that has passed its sell-by date because it is the roadblock to their fantastical dreams of as yet unknown far-away trade deals. These former prime ministers, along with soft-Brexit MPs, are accused of “using” the Irish question for their own pro-EU purposes. But Boris Johnson’s cavalier dismissal of inconvenient Irish peace is the grossest abuse of a foreign secretary’s responsibilities in our lifetime.

The Daily Mail retaliates against Major as the most “hopeless and hapless” prime minister of our times and Nadine Dorries calls him a “traitor”, no less, for delivering the strongest speech of his mostly insignificant political career. Full of newfound conviction, Major calls for parliament to decide and if not, a referendum on the final deal. Blair too was at his most reasoned and persuasive, heading for Brussels to try to ease the freedom of movement impasse.

What influence do they have? These two has-beens serve only to remind us of the inadequacy of our present leaders in this national crisis, as Theresa May finally reveals on Friday what her Brexit means, including for the Irish border. Or will she parade another nest of unicorns of impossibility?

Blair in Brussels seeks easement on freedom of movement, pointing to rising EU nationalist and xenophobic parties. Observers say no chance: he is disingenuous in blurring EU sentiment against arriving Muslims from outside, while internal freedom of movement is largely uncontentious. But he can bring back reminders that other countries such as Belgium use restrictions within EU rules that neither he, David Cameron nor May made use of: registration of all new EU arrivals, sending people home if they have no job after three months – which needs universal ID cards – and insistence they hold insurance for health and unemployment.

Sinn Féin need only appear for the few crucial votes that would stop the Brexiters wrecking Irish prospects

Those might be enough to sway leave voters who now see the inescapable real world consequences of Brexit all around us, as the high street is hit by an avalanche of shop closures due in part to falling living standards, less shopping, fear for the future and the plummeted pound, while the NHS loses nurses and GPs. All this people start to see already, while the rest of the EU surges ahead of us.

Each week negotiations conducted with grudge and blame by pig-headed and pea-brained obstinates reduce the prospect of damage limitation. But of all issues, the Irish border was always the boulder there is no climbing round.

Dishonestly, some Brexiters shrug, and say let the Irish and the EU put up their own border if they want: that’s their funeral. That’s nonsense: World Trade Organisation rules require member countries not to discriminate between their trading partners – so if we went for no border with Ireland in a no-deal Brexit, we would need to be equally open to all the world for everything. As a “no deal” threatens, the game is beginning to change to who the public will blame most afterwards.

But since almost every step forward is checked by impassable red lines, all red lines may be up for reconsideration – even one fixed since 1917, the question of Sinn Féin’s seats in parliament. The taoiseach, Leo Varadkar, ventured to call on the party on Wednesday to take up their six – soon to be seven – Westminster seats “to make things better for Ireland”.

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He’s right: this should be their heroic moment to step up to rescue their country, north and south. The Irish Labour party joined the call on them to “defend the interests of Ireland”. If only they could mutter the loyal oath (they could always rescind it later) they would arrive in parliament as a cavalry of saviours of Ireland – and incidentally Britain. As a counterweight to the Democratic Unionist party votes, they could ensure a soft enough Brexit with customs union and single market to keep the border open. They need only appear for the few crucial votes that would stop the Brexiters wrecking Irish prospects – then they can retreat again as noble deliverers of their nation.

Naturally, like all other red-liners for now, they say no. A spokesman says: “This is not even a topic of discussion within the party. We are an abstentionist party, and we are mandated to abstain from Westminster by the people who vote for us.” For now, everyone says no. The winners might be those who make the biggest U-turn sacrifice for the sake of their country.

• Polly Toynbee is a Guardian columnist