You are missing a truly simple point; why on earth would the EU give us a better deal on the back of a lost Parliamentary vote? What is in it for then? Acclaim from Vince Cable, Anna Soubry and Keir Starner? We will be made a take it or leave it offer, which Parliament are free to accept or reject.
You're not understanding the dynamics here. HMG can't agree amongst itself what they want.
If HMG fails, then Parliament will decide.