The EU should offer the UK an extension up to the end of their fiscal period, 21 months, but let the UK leave at any point up to that date, the remaining period being then a transition period or managed no-deal period, the same money paid into the EU regardless. It sort of gets the UK away from a cliff edge exit and keeps the EU in easy money for an agreed period. At worst, the UK just pays what it would pay as an EU member for access to the single market. The problem of the UK not having any MEPs would have to be fudged with some agreement between all parties - the general public could not give a shit about MEPs, as they are seen as being powerless in comparison to the bureaucrats and leaders of individual countries. Being still in the EU we would have a veto on all the important stuff.
The ERG and DUP, as soon as they solved the Irish border issue, amended the withdrawal agreement to suit, could then get us out of the EU. Meanwhile, because the backstop is so interlinked with the trade agreement, everyone else can use the 21 months to finalise the trade deal without actually singing off the withdrawal deal - takes power away from the EU but about bloody time! Once the trade deal is done and dusted the backstop doesn't really matter as a trade deal that allows frictionless trade is the only option to keep the Irish border open.