No I don't think at the last second we will back out of this, because if the Tory's do this they will never ever be able to stand for Election every again.
Inbetween everything else, this is the key and most relevant point, I think.
You've said the Tories won't back out because of political reasons, which I guess means you know in theory they could. I think most people who've looked much at this get that it's a process that could be stopped, so that takes us then to the question of whether it would be.
So first of all that presumes it'll be the Tories decision. Which is interesting because they don't actually have a Commons majority even now, and certainly on the most recent data, they'd lose seats to Labour if there were a GE called now. Obviously there might not be another one before March 2019, or if there is the Tories might improve their position, but on our current best information, they won't. There are also likely to be by-elections before then, which historically doesn't usually favour the governing party.
The Tories are reliant on the votes of the DUP, and bear in mind also that Ken Clarke won't vote for any Brexit deal. So that's 315 Tory votes max, unless they can manage to win some by-elections before then. As Sinn Fein don't take their seats, they'll potentially need a majority of 643 MPs, so 322. They will be reliant on the DUP.
Now remember the DUP simply won't have a hard Brexit. For all the posturing, they know how fucked they'll be if the border issue isn't sorted out. You saw how hard they bargained for something as relatively straightforward as cash. There isn't a chance in hell they'd vote for a Brexit that didn't settle the border question to their advantage. How likely is that to happen? This is before we even begin to consider the Lords.
And the thing with Tories is that, as you sort of allude to, they're pragmatists. If we don't get a decent deal sorted out in 18 months, which we're not going to because trade deals take a long time even with the best will in the world, they'll be presenting the population with a turd. A turd that will make us poorer. If you know politics, you'll know how quickly Conservatives historically disassociate themselves from that kind of thing- remember Thatcher and the poll tax? They've never been big on ideology over practicality. It's why they've managed to hang around for so long.
None of this is to say the EU is wonderful, or that it isn't potentially possible to design a mechanism and circumstances that might make Leaving to be of benefit to us. But those aren't the mechanism and circumstances we're dealing with now.