Never underestimate David Davis. Dismissed by many socially grander Tories, he remains defiantly at the heart of the action. Often attacked as an incompetent Brexit secretary, he was, for instance, one of the first cabinet ministers to grasp the need for significant exemptions to the new immigration rules if the British economy were to stay afloat.

Most tellingly it was Davis, rather than Boris Johnson, who resigned first over the Chequers deal. While the foreign secretary (as Johnson still was) lingered in the dressing room, preparing for his grand stage entrance, his cabinet colleague got on with it and did the deed. You don’t have to share Davis’s views on Brexit (and I don’t) to see that, of the two, he behaved more honourably.

Johnson is now reportedly furious that Davis is considering another run at the Conservative leadership (he was defeated by David Cameron in 2005). But – again – it is Davis who, writing in the Sunday Times, has taken the initiative and called on his former cabinet colleagues “to exert their collective authority”. At this week’s EU summit in Brussels, he continues, “the authority of our constitution is on the line”.

It is time that Brexiteer MPs and the DUP fear … What is agreed in 2018 will look very different in 2028

Three such ministers – Andrea Leadsom, Esther McVey and Penny Mordaunt – are known to be considering their respective positions. For Mordaunt, in particular, it is a difficult call: the international development secretary’s political stock has soared in recent months. She is that most rare phenomenon – a modernising Brexiteer, as likely to be championing trans rights as defending the result of the 2016 EU referendum. Does she broaden her appeal to pro-Brexit MPs and Tory members by quitting over a prospective deal that is very unlikely to please her (or them)? Or does she, like Michael Gove, stay at the top table and bide her time?

There is a gathering rebellion on the Tory backbenches over May’s proposals for EU departure: 63 Conservative MPs have signed a letter to Philip Hammond, complaining about leaked Whitehall forecasts of the adverse impact Brexit will have upon Britain’s GDP. But the principal objection the leavers have to the prime minister’s plan is its provision for a “backstop” mechanism that would keep the UK in the EU customs union until the Irish border question is resolved.

Facebook Twitter Pinterest ‘Three ministers – Andrea Leadsom [left], Esther McVey and Penny Mordaunt [right] – are known to be considering their respective positions.’ Photograph: Rex/Shutterstock

Meanwhile, and no less significantly, it has emerged that Arlene Foster, the leader of the Democratic Unionist party, believes a “no-deal” Brexit is the most probable outcome of the talks. What Foster opposes most vigorously is the proposition that, to avoid a hard border on the island, Northern Ireland should stay in the EU single market as well as the customs union – leading to what she calls its “permanent annexation away from the rest of the UK”.

It is powerfully clear that the DUP’s readiness to prop up the Tory minority government is being tested to the limit. Indeed, Hammond’s budget on 29 October may yet be the occasion of the government’s fall. Foster has made it unambiguously clear that May and the chancellor cannot take her party’s support for granted if the contours of Brexit prove to be as “dodgy” as she suspects.

Time is one of the most powerful forces in politics: habituation breeds consent, or at least weary acquiescence. And it is time that Brexiteer MPs and the DUP fear. Rebellious Tory backbenchers fret that Brussels will contrive to keep the UK in the customs union, and that a notionally temporary arrangement will become, in reality, a permanent one. Foster’s Unionists see a more elemental danger on the horizon: namely that Northern Ireland, detached from the UK and still in the single market, will slowly become, in practice, part of a mostly unified Ireland. And this much is true: what is agreed in 2018 will look very different in 2028. Time will have its way.

Which is why we are entering the most constitutionally tense period in British history since the second world war. In spite of premature reports in the German press, it is unlikely that this week’s European council summit will settle the terms of Brexit. But both sides have higher hopes for the additional meeting pencilled in for mid-November.

Are Brexit talks finally about to yield solution to Irish border impasse? Read more

If May returns brandishing an agreement next month, she will face a moment of profound paradox: having achieved a historic deal, she may be rewarded by political annihilation. Under section 13 of the European Union (Withdrawal) Act 2018, parliament is entitled to a meaningful vote on the outcome of the negotiations. If MPs reject that deal, the government has 21 days to make a fresh statement to the Commons about its intentions. Next, the prime minister has until 21 January to present an amended agreement. And after that? Well, then things start to get really vague.

Over all this loom the twin zeppelins of a snap general election – yes, another one – and the People’s Vote, for which campaigners will march from Park Lane to Parliament Square on Saturday. In her conference speech, May showed that – unlike many of the more bombastic Brexiteers – she takes the pressure for another referendum very seriously: “If we don’t [unite] – if we all go off in different directions in pursuit of our own visions of the perfect Brexit – we risk ending up with no Brexit at all.”

With each passing day, the shining palace that leave promised us in 2016 looks more and more like a dilapidated Victorian folly. Hectically, crazily, the nation is careering towards the unknown. I’d say it could go either way – if it were remotely clear what those ways might be.

• Matthew d’Ancona is a Guardian columnist