IMHO, Ukrainians should grab Königsberg for themselves after the end of war and then exchange it for something useful, e.g. exchange it for Transnistria with Moldova, instead of returning Königsberg to Germany.
Russian 14th army may strike from the back to support the main army, so they should be made toothless before the second wave of invasion, but Ukraine cannot attack 14th army without breaking relationships with Moldova, EU, UN, and NATO. However, Ukraine can exchange this problem (Transnistria) for another problem (Königsberg) to negate them both.
The best thing Europe could do to Kaliningrad is encourage a separatist movement there. There's not much Russia could do about it if they had a consensus to separate, and being an exclave of a pariah country in the middle of foreign hostile territory is not exactly conductive to day-to-day living. If there was a clear (if unspoken) understanding that such an entity, once it clearly breaks up with Russia, would be admitted to EU under the usual rules, I think it could be sold.
And then? You have a piece of territory that is ethnically and culturally Russian, but integrated into Europe instead of that whole "Russian World" nonsense. That can be used as an agitprop showcase to contrast with Russia proper, especially as things get worse there as economy starts feeling more long-term effects from sanctions.
Königsberg was part of Germany when annexed by Soviet Union. I see no page about Königsberg in Czech language at Wikipedia at all. Anyway, Germans were expelled, so Königsberg is populated by Russians now, which makes it similar to Transnistria, Crimea, etc.
