Parlimentary Remainer end game

I don't think you've misunderstood anything, as with everything to do with Brexit it's a bit of a mess. As for wanting a vote, there's multiple motives at work here. 

Some MPs were possibly motivated by genuine belief in promoting Parliamentary sovereignty, it's a big idea in U.K. governing dating back to Davey (scholar who's considered a big authority on the UK constitution) in the 1880s. 

Some more likely want to ensure the opportunity to have a chance to influence Brexit, as in order for this 'meaningful vote' to take place a bill must be placed before Parliament. As with every bill, there is the possibility to propose amendments. Crucially this would give the Opposition another chance to try and influence Brexit in some way, even if only through a minor amendment which is still better than nothing. This likely explains Labour's support for the rebellion. 

Some MPs might hope this pushes the government towards a 'softer Brexit', as the Government will presumably try to make a deal that is likely to pass Parliament (for the deal to fail would be a monumental embarrassment for the government) so will seek an arrangement likely to please the majority of MPs-pushing it further away from minority hard Brexiteers' vision. 

Are there some determined Remainer MPs who think this vote might be a backdoor way to cancel Brexit completely? Possibly, though I can't see how that might be achieved. By voting for initiating Article 50 MPs have already okayed leaving, so I can't see how voting against the deal undoes that. 

What if the deal is voted against? Well it depends when this vote is held, expectations seem to be for March 2019-same month as the Article 50 deadline. Now if it were to fail, I think the possibility to extend the deadline does exist (I can't remember where exactly I read that though) however the government is at the moment pushing for an amendment to the European Withdrawal Bill to legally set the date as 29 March. If that amendment passes, and then this deal fails-the UK crashes out of the EU with no deal in place. And in that case WTO is the only option. This is why a rebellion to stop the government's amendment (possibly next week) is also likely. 

I don't think the agreement is in danger of actually being voted against unless whatever's agreed is really toxic. I remember much speculation following the Gina Miller Supreme Court case last year that Parliament would block Article 50 and in the end that went through without a hitch. 

I've posted before, that as far as I could tell, once Article 50 was invoked, parliament and the people were just along for the ride. Sure, in theory both parliament and the people should be able to influence the deal, but that kind of influence takes time, time doesn't exist in this case. We will get (I believe) whatever deal the government is willing and able to negotiate and parliament will have to take it or leave it... and I'm not even sure they would get the opportunity to leave it (reject the agreement) as I expect any government, even a new one formed in the wake of such a humiliating parliamentary defeat would see little option but to move forward with the agreement in order to minimise the harm done the UK.

As always, I must stress I do not understand a lot of what Brexit means. I actively avoid the news a lot of the time. It depresses me. But you can't escape Brexit!

I read conflicting reports that do nothing other than deliberately obfuscate the issue to their own agenda (it seems to me). The Government are using the upheaval of leaving the EU as an excuse to push forward certain laws and legislation that satisfy their own policies rather than what was voted for, I read. However, that viewpoint could be being used by a media that has an anti-Government agenda and is seeking to undermine them. Elsewhere, an American report (I think) has predicted the UK's economy will be 4.7% worse off than it is now - which although certainly not desirable, is hardly the maelstrom of disaster predicted by those who don't want Brexit.

So what do we believe? Or should I stick my head back under the pillow?

Yes, the majority of the British population is made up from various immigrant waves that have overwhelmed the indigenous people of the islands, the first such wave contained many different people from across Europe and Northern Africa, the second major wave, and whose languages and customs now dominate were a French-Germans, followed by Scots from Ireland, since then there have been significant waves of immigration from the Nordic countries.  The leadership classes (nobility etc) were almost to a person taken over by immigrant French, who were in turn immigrant of Nordic decent.  There have also been significant influxes of Irish from Ireland and Scotland, Jews and Roma both from Asia, Huguenots from France and Indians from the Indian sub continent and all that took place prior to 19th century.

Since the start of the 19th century there have been significant immigrant waves of Indians, Africans, Germans, Russians, Arabians, Belgium, Chinese, Cypriots, French, Hungarians, Italians, Jews, Poles, South Asians, West Indians and around 1 million peoples whose race is classified as "other".

It's a bit like the US in that respect, the majority population are from immigrant stock.