I don't think "Corbyn as PM" is a realistic option. It gives political benefit to a caretaker leader, and much more benefit to one leader than the others in a "Unity" government. I see it like when you get a "caretaker" leader for a political party after one leader steps down before the next is chosen. In that case, you find someone minimally controversial, with no ambition to lead in the long term. Corbyn is ... not that.