Isn't a border in the Irish Sea a far worse outcome than the backstop as far as Biris and the ERG concerned? So his plan is to negotiate a worse deal.than May's that they all hated and voted against?
The Brexiteer extremists are super paranoid that there will be a second ref unless they get this done quickly, and a lot of Labour MPs are fearful of a no deal unless a deal is reached. Others from Leave constituencies also fear a second ref.
Feels like Boris is gambling that fear will get this through parliament regardless of what the DUP do, and that he can hit his promise to leave by the 31st. Most Tory voters won’t care about NI being sold down the river and he’ll be the guy who delivered Brexit. Even if the deal is rejected by parliament he can run a GE on Brexit Boris vs Parliament against the will of the people and likely win.
If MPs want to stop him, they need to vote this shit down and immediately get him out of office with a GNU in place. Probably won’t happen, but right now his position is looking fairly strong otherwise.