Also are there any attempts by the Polish authorities to recover these national treasures
No, there`s no way and no Polish government has ever tried to. The reason is simple: when Sweden attacked and occupied Polish territory, it wasn`t solely Poland but the Commonwealth - Poland united with Lithuania. Formally, today`s Poland has no right to represent the Commonwealth in court. Also, Tacitus is right about aristocratic or royal property.
some of which are now in Swedish museums.
A Swede tried to explain it to me. It doesn`t matter in which museum they are - Polish or Swedish. What matters is that they didn`t disappear or get destroyed during all wars, risings and occupations which happened in Poland during next 300 years after the Swedish Deluge. They have been preserved as the heritage of European culture and that matters most.