Was the citizenship of the citizens of the Second Republic a right, or something less than could be taken away from them involuntarily?
It was not a right, it was a privilege which could be taken away and was taken away.
If it was a right, then the whole Yalta border change couldn't (under present E.U. human rights law) deprive those in the East of it either,
Even if it had been a right and so somehow it couldn't have been taken away due to a law which hadn't even been written at the time (helpful hint: it wasn't and E.U. human rights law was years away at the time of the Yalta border change), a person who wishes to claim that their ancestor was able to pass on Polish citizenship to another one of their ancestors would still need to be able to prove that that ancestor had Polish citizenship to pass on to another one of their ancestors. The burden of proof is on the applicant, not the state! If what you want were to come to pass, every person in Africa and Asia would be able to walk into the nearest Polish consulate and say "You cannot prove that my grandfather was not a Polish citizen. I say that my grandfather was a Polish citizen and you must therefore grant me Polish citizenship." If you can't prove you have a claim to Polish citizenship, you won't be granted Polish citizenship, it's as simple as that. Just as a person who can't prove they do not owe taxes to Poland will not be granted citizenship or a residency permit.
Try Ukraine instead.
That would offer visa-free access to the EU and simplified work permit procedures in some member states, or at least it does to those who don't have black marks against their name for over-staying in the Schengen zone.
