I do not know why he never returned to Poland.
The immediate post-war period in Poland was a very difficult time, some of the returning soldiers, especially those from affluent or educated backgrounds were put in 'labour battalions' which were effectively prisons. Some were executed.
If that decision was incorrect can It now be reconsidered and corrected or is there a statute of limitations that would apply in these circumstances
If probate was signed off at the time, it might (and should be) be too late. You also need to consider that it's been someone's property now for three generations or more and they would be appalled at a distant relative (that they don't know or consider to be family) from another country trying to take it from their family on the basis of a pre-war will. Worth putting yourself in their shoes here.
And what would you do with these people's land if you actually got it back? Move to Poland and take up farming?