This was originally just pure speculation, but I’m starting to think that it might actually be a plausible strategy for Boris Johnson to pursue:
As quickly as possible, Johnson makes the following (confidential) proposal to the EU:
“I still believe firmly that it it is feasible to put in place frictionless border checks that preserve the open Irish border. But the current backstop – which forces the entire UK to comply with EU regulations until you approve the border arrangement – is simply not politically feasible, regardless of whether I support it or not.
However, I’m prepared to jettison May’s red line of no hard border in the Irish Sea, and make you this alternative offer:
– The UK mainland can immediately start diverging from EU regulations once we leave.
– Until such time as we find an acceptable arrangement to create a frictionless land border (which I hope and believe will be as soon as possible), Northern Ireland will remain in regulatory alignment with the EU.
– If this creates the need for border/customs checks, they will take place in the Irish Sea between Northern Ireland and the rest of the UK.”
If the EU reject this, then Boris has grounds for painting them as villains for refusing his substantive offer, and use this to pursue No Deal.
But unless they simply don’t trust the UK to honour this deal, I see no reason why the EU wouldn’t accept this offer, since it gives them what they want (an open Irish border + border checks against areas outside the EU) – and the Irish government might even see this as better than the current backstop, since it provides (more direct) impetus towards Irish reunification.
For the same reason, this arrangement will obviously be utterly unacceptable to the DUP (irrespective of how much Johnson insists that these border checks on a sea border will be completely unnoticable), who are likely to not only block such a deal, but also bring down any government that proposes it.
So, with no prospect of any other government having a majority in the current parliament, Johnson will presumably be able to dissolve Parliament and call a general election (and get the EU to approve yet another brief extension), where Boris will be campaigning on a platform of “I have a deal on the table which both Downing Street and Brussels have signed up to, all I need is a Conservative majority in Parliament to get Brexit over the line and fulfill the People’s Will &c &c” – trusting that
a) this will undercut the Brexit Party’s attempt to split the Tory/Leave vote
b) the large majority of Tories (admittedly among members rather than all voters, but still) who prioritize Brexit higher than preserving the Union (plus those lukewarm Remainers who decide that this Brexit is preferable to perpetual limbo and the threat of a no-deal), will be enough to win them a clear majority and sideline the DUP.