Parliament triggered A50. If they and every other regional, national and euro parliament don't approve whatever deal is submitted, hard brexit it is. Not my desired outcome, I would rather the EU were reasonable but they aren't so we are where we are.
I don't believe that is the case. If HMG fails in this venture (and it's not doing very well, either deliberately or through incompetence) then I would expect Parliament to sort it out in the national interest.