But I'm talking about the (entirely feasible) scenario in which Corbyn stepping aside for one of yer Benns or yer Coopers would be the only way to *avoid* No Deal.
What would be their justification then?
They wouldn't need a justification. They could quite simply state that Corbyn is Leader of the Opposition and elected leader of the Labour Party and that it therefore falls to him to form a Government and that none of his MPs has any mandate whatsoever to do so.