“But if countries show that having refugees on European soil – … – is not a worst-case scenario but a perfectly manageable situation, they remove one more weapon from Russia’s arsenal.”

That is true if Putin had intended to browbeat Europe to in turn browbeat the Ukrainian government into caving in to Putin’s demands. Somehow that seems a fanciful reason for Europe to console itself that it is fighting the good fight by rolling out anew, as well as hollering out for, the multicultural society. Because, as you also say, “The relentless Russian targeting of civilian and vital infrastructure in Ukraine – … – seems calculated to make Ukraine uninhabitable, thus forcing an unprecedented number of people out of its borders.” That has been the chief aim, the consequences be damned, of the Russian invasion: to drive away Ukrainian-speaking Ukrainians from the east of Ukraine in order to stamp a Russian claim to those vast swathes of territory.

As it turns out, many Russian-speaking Ukrainians, especially the older, must be torn. They don’t want to abandon their homes in the war zone, yet are fearful of heading west, deeper into Ukraine to escape the war on account of their fear of being discovered to be Russian and from the east. And they would not want to flee to Russia either. So perhaps some have decided that they might be not so badly treated by Russian soldiers and will chance remaining in their hometowns.
So perhaps Putin realises that, too.

A lot of conflicts, once they kick off in earnest, take on simple aims. The finessing of strategy goes out the window. The Muslim-minority Rohingya in Burma or Myanmar or whatever it’s called these days, were burnt out of their homes into refugee camps in Bangladesh. Presumably the land they had lived on is seen as rightfully belonging to the ruling majority.

The half-million Yazidis of north-central Iraq were run out of their old lands: but they clamoured to fight back against the violent zealots from safer Kurdish territory. If they were to lose their grip on their old lands, it would have been the end of the Yazidi people (like a diaspora Jewish population before the creation of Israel). A mass swift repatriation outside the region would have been doing the Yazidis’ enemies their bidding, effectively.

And then there was the swift, cynical passage of mainly anti-Assad Syrians and Kurds through the mass of Turkey onto rickety boats to cross over to Europe. It was the emptying of undesirable people from their war-torn lands being fought over by powerful people, rivals, hoping to extend their power and influence.

What those examples show is how utterly futile it is to imagine that the formation of functioning fully-multicultural societies in Europe is going to be an effective riposte to the selfish tyrants outside Europe. If they don’t end up functioning societies, and fear and chaos ensue, with bitter self-recrimination into the bargain, then the tyrants are laughing: they’ll feel unexpectedly extra powerful as well as even self-righteous in their own nationalistic outlooks. If, somehow, Europe comes through it and everyone pulls together, including the newcomers themselves, then that is a greater incentive for tyrants to open more fully their own borders and invite whoever undesirable or surplus to requirements is left to leave.
In other words, whether Europe faces the coming decade with grim determination (and taxes the rich very heavily) to manage roofs over the heads for everybody with adequate kitchens, or not, it’s a win-win situation for hard men leaders outside the civilised West.

Putin may eventually claim that his attack earlier in his invasion directly towards Kiev from the north was to make it seem as if the whole of Ukraine was under threat. (It still is!). And that threat would ensure a greater amount of Ukrainians fleeing overall, especially from the east. As the Kremlin sees it, only good Russians and good people deserve to stay in their future utopia. Those who would acquiesce to them.

On a final note: have the non-Ukrainians who fled Ukraine without their passports? Their countries have embassies in The Netherlands. Well, maybe it’s all hands on deck to show that Putin that Europe CAN do it!