others may feel that it's in their national interests to make things difficult
Can anyone think of a country of any particular economic power that would think that making things difficult for trade with the 5th largest economy in the world was in its national interest? This is a prisoners' dilemma, tragedy of the commons thing: so long as all the EU countries hang together they might possibly get away with it with their voters, but the moment one country defects, the whole thing comes tumbling down.
Firstly, the tit for tat stuff is asymmetric. Suppose Poland and Romania were co-erced into playing hardball over freedom of movement. OK, say the UK government, all your citizens can fuck off: as of Monday morning, it will be illegal to employ a Polish or Romanian citizen without a Tier 2 visa, and all benefits, tax credits and access to schools and the NHS will cease. Planes leave from Stansted, don't let the door hit you on the way out. What's their response? That the vast coterie of UK expats living in Poland and Romania have to leave? All ten thousand or them, or so? What's the ratio, a hundred to one?
Similarly, suppose Germany plays hard ball and makes access to German markets difficult. OK, no problem: we'll remove all tariffs on Japanese cars and sell them in the UK at global prices and we'll make the same offer to any European country that fancies it, so if France fancies being a bit more understanding, lots of sales for Renault even if the electrics are still manky. We would simply approach individual countries and offer them individual deals, and if they want to refuse them for the good of the collective EU, we'll simply tell the world and watch them deal with their voters.
Secondly, it's hard to think of an EU government right now that has any love for the EU, aside possibly for Germany and even then the voters are in open rebellion on it. If an EU nation says "OK, we'll torch a few bits of our economy to take revenge on Britain and pour encourager les autres", what is the effect of that going to be on their local electorate? It'll get Le Pen into office in France, for sure, and then things really start to unravel.
Again, for clarity, I'm a fervent remainer. It's just that I despair at how hopeless my companions' arguments are.