Another point Mr Kerr, how can you say the agreement won't be opened up again? I will put a scenario to you- one you won't like, but still-lets say there is a no confidence vote which leads to an election and horror of horrors for you Labour win comfortably. The EU will be dealing with a totally different kettle of fish then why wouldn't they at least be willing to reopen partly because it might suit them better a Labour brexit than a tory brexit.
In a similar way maybe opposition parties could force the govt to renegotiate or failing that have an interim govt of their own that does.