I think the whip will require all the conservative MPs to vote with the government. If an MP defies a whip, the consequences can be serious - particularly a three line whip, so I think more MPs will vote with the 'deal' (which isn't really a deal yet) in the end. But MPs do defy the whip so I couldn't say for sure.
The DUP are the ones that could put the spanner in the works as they can reassess their confidence and supply deal, which Arlene Foster has said she will do.
The 'deal' is not really a deal, it is a document putting preferences in for the deal. The withdrawal agreement is the 500 page doc that is a legal document. It basically makes it a smooth exit and buys both of us (EU and UK) time to make another 500odd page doc outlining the actual deal - end point 2020 I think. If no deal is reached in that time, then we enter the dreaded backstop which is what everyone hates - nobody, including the EU, wants to enter the backstop as that is just dragging things on and has some economic consequences for NI, as well as this border issue, although I think that is a bit of a red herring.
I cant remember if Arlene Foster (DUP) hates the withdrawal agreement and the backstop, or just the backstop. If there was some way of separating the two, perhaps she would be happier, I can't remember.
It is a good story to say it won't get through parliament but the whips will have other ideas, unless MPs are given a free vote, which I cannot imagine they will be. But I don't actually know that at the mo.
If it doesn't go through, Theresa may go back to the EU and negotiate again, and perhaps get the things that are rubbish sorted, although I wish she would do that now - although the EU aren't playing that game at this stage I don't think. Saying that, they could be talking behind closed doors. Perhaps she will say, look I can get the withdrawal agreement through, but not the backstop - which is the 'safety net' (which people def don't like) in the event of no deal then being arranged.
I cannot say for sure obviously, but I highly doubt the occurrence of a 2nd ref or a GE (there won't be the majority for either, the Tories won't put the option of a 2nd ref through the house, not a chance as I see it, and there is no other way to get one, and after last time there is no way May will call a GE lol) - if anything, it'll be a leadership challenge - it which case I think there is 14 days where the Conservatives have to get behind the new Leader which the party, and the DUP, then have to vote for - there has to be a majority backing the new leader/govt/cabinet. Then perhaps, if it was someone like Boris who is a bit kamikaze, we would go out on WTO without a transition arrangement/withdrawal agreement, which we would have under Theresa May, ...but then we would have to negotiate a deal anyway.
We cannot negotiate a deal until we are out, after March, it is not legally possible, so we have to come out first, and then sort the deal. The difference is that with Theresa May's 'deal' (although it is not a deal as such), we will have a transition period so that we don't have full on WTO rules - basically starting again, where there would be more chaos I would think.
Anyway, either way we leave, it is just how we leave. And Labour, and others, mucking about and saying we want a GE or a 2nd ref is just sabotaging the process and making it worse imo, not to mention getting people's hopes up perhaps, not to mention them standing on a manifesto saying they would act on the referendum. But its a good story lol and gets them on telly perhaps. Anyway, we leave in March, and ideally, we need a transition period of time, which Theresa May has managed to get legally agreed, if we vote it through. I think actually you will get more MPs voting it through than the media are saying - wouldn't make a good story though would it, across all the parties, as coming straight out on WTO is a total ballache. Doable, but more of a ballache that having time to then negotiate an actual deal.