The EU has been fairly consistent that if the UK changed its mind they would be very supportive of this -- which, I imagine, won't come as a shock to Leave voters. Which means that, yes, it is rather up to what Parliament decides. I think the current expectation is that the Deal will be voted on at least twice, being rejected the first time, and maybe also the second time. In that case, I suspect Parliament would probably move either (1) to have No Confidence in HM Government, or (2) to request a suspension of A50 while working out where to go next.
All of this is moot, for the time being, but, the sovereignty of Parliament being what it is, there may be mechanisms to annul the process.
At least one step might be to repeal the EU (Notification of Withdrawal) Act 2017. Since it was deemed necessary to have such legislation to grant a notification of Article 50 in the first place, it may be correct to say that repealing the Act withdraws the permission Parliament had earlier granted. I *do not know* whether this argument is legally correct -- but, if it is, then the wording of Article 50 might imply that our notification is no longer valid, as continuing to view it as active might violate our constitutional requirements.
At the moment, though, I tend to expect that Parliament will eventually, after a great deal of fuss, accept the deal on offer. We'll see.