Reading several of your arguments, I don't really see how they contradict the idea that Putin has been working to destabilise the EU and therefore Brexit is a boon to him. Sure, he didn't have to go to much trouble, and there has been a lot of crap policy in the UK and elsewhere that facilitated his aims, but he has nonetheless been tinkering away at them for years.
Any smart political leader with geopolitical ambitions will be ready to take advantage of instability to advance his or her interests. This has been an overt part of American policy since the days of Land Lease. Only recently, under Trump, did the US disavow its policy of regime change in countries where it suited its interests, which it has followed since WW2.
The UK did the majority of the spadework in destabilising the EU by advocating so strongly for the admission of eastern European countries, at the behest of the US, who saw an opportunity in the weakness of Russia in the 1990s. Those countries for the most part do not share the democratic traditions of the west (sometimes the western democratic traditions are pretty new-found and shaky themselves). The US under Trump was hell bent on demolishing the EU, for its own reasons, and actively supported Brexit, with many big donors to Trump, and Cambridge Analytica/the Mercers playing an active role in Trump's campaign and the Brexit campaign. The captains of industry who threw money at Trump have not gone away and still hope the EU can be dissolved, clearing the way for American agribusiness and hedge funds to carve up the pickings. They also want Ukraine, btw.
I'm not as optimistic as you about all Eastern European democracies. A fair few are shaky, several have been through recent crises, resignations and elections and their governments are divided and young.
I'm not optimistic at all about eastern European democracies. The alliance of nationalists and conservatives doesn't bode well in Poland or Hungary, while Romania and Bulgaria are barely integrated into the EU economy.
Putin's remarks about returning to "pre-1994" borders are alarming from that point of view, even taking on board the lack of realism underpinning his rambling speech earlier this week. Although, from what my sources in one E.European country tell me, those remarks have had the effect of focusing minds and reducing division pdq.
I am not optimistic about the long term effects of any resurgent nationalism in eastern Europe in countries that are most likely to be affected. Resurgent nationalism in eastern Europe is not going to be forward looking. It will ensure that the loudest voice expressing nationalistic views will get votes regardless of what else that voice shouts - an end to women's rights, denying human and civil rights to immigrants and asylum seekers, Roma/Sinti minorities, speakers of minority languages within borders, whatever...
To a large extent, Europe is still debating the question of the important founding event of the modern era in Europe - is the starting point the French Revolution or the Congress of Vienna and the Holy Alliance? A lot of eastern Europe (and Russia, continuously since 1815) says the Congress of Vienna and in particular the Holy Alliance, with its emphasis on stability above ideals, particularly if those ideals are liberal/progressive. There is a lot of sympathy for that stance in Poland and Hungary. This doesn't mean P and H will collapse. It does mean they may not feel very much at home in the EU.