I'm afraid I don't understand why you think that chart supports your argument so strongly. Although it does show that the views of remain voters about leave voters are distorted, it nevertheless indicates that immigration issues were actually a (slightly) stronger factor than sovereignty for leave voters. Even if immigration were slightly weaker than sovereignty, it still doesn't change the fact that immigration was the key motivator for a substantial fraction of leave voters, and certainly doesn't support the argument that "likely all Leave MPs are primarily motivated by sovereignty issues".
Of course this doesn't mean that everyone concerned over immigration is a racist, but it certainly doesn't support the view that immigration concerns were of far lesser significance than sovereignty.
I admit also that things have changed since then, with many people now primarily motivated by seeing the result of the referendum enacted to ensure that democracy functions as intended, and I think sovereignty is wrapped up in that now that the EU is seen by many as interfering with the UK's democratic choice.
EVNow has a chip on his/her shoulder that Brexit is an overwhelmingly a racist project. What is clear is that less than 40% of Leave voters even mentioned immigration as their primary voting factor, and as you note, an unknown portion of this group really meant they wanted sovereign control of immigration rather than to stop immigration.
Speaking for myself and many people I know, I find the immigration system necessitated by EU membership to be inequitable and racist. It says that if you are one of half a billion people from one of 30 European nations (which are predominantly white and christian), then you have the same rights as national citizens. But if you're from any of the other "scarier" parts of the world (be it Africa, India, China, or even the US, Canada or Australia), the immigration system is a minefield.
Further, the current system has encouraged immigration flows which have worked very nicely for the educated professional class in the UK but quite badly for the rest. Since the Eurozone crisis, the UK economy has been an engine for private sector job creation and this has continued even after the Brexit vote (1 million extra jobs created since the 2016 referendum). But wage growth has been very poor, particularly so at the bottom end of the income pyramid. Real incomes are still below the pre-2008 level. It's not a stretch to think that expanding the potential supply of labour by half a billion people (including from some far lower income states) has capped low skilled wages. When you're university educated and in such a privileged position that you can invest in single stocks such as Tesla, it's easy to throw stones at what you see as the racist plebs that don't like immigrants but this somewhat misses the bigger picture.
In terms of my confidence in the rationale of MPs, these are people that are so obsessed with the act of lawmaking that they have chosen to pursue a parliamentary career. The current UK parliament is on the whole quite remarkably free of political extremists and it's ludicrous to think that xenophobia outweighs their desire for preserving the sanctity of parliamentary sovereignty. It's not just Brexiteer MPs that find the Backstop democratically unconscionable but Remain supporting ones too. And as you mention, there are plenty that think Brexit is the most idiotic idea they've ever heard of but who now back its implementation at any cost, as they understand that losers consent is the pillar of any stable democratic society (see Noel Gallagher above).
It's this fundamental mis-reading of the underlying motives of the Brexit project, by both the previous UK government and those setting the European negotiating mandate, that have led to the current gridlock. This failure has now been remedied on the UK side but the European side seem irretrievably unreconciled, which is why a No-Deal Brexit is by my book now a racing certainty.
