Stolen land is when an international dictate / decree forces you to hand over your villages to your enemy.
After that, all murder and robbery is fair game, by all national administrations.
If you restrict people's movements, then you just reinforce such military problems.
Especially when you put it back in the hands of national governments, which started this problem in the first place.
Also, under such a system, some of those closed nations must end up dirt poor and some others filthy rich, because trade always consolidates, and you have no central Brussels bureaucracy that can participate in this network to balance it.
National problems can flare up as long as nation exists. Will the EU be able to assimilate European nations and create the one nation of Europeans? I highly doubt that.
Yes, some nations will be poor, some will be rich. It will all depend on their competitiveness, creativeness, education systems, the quality of management, and so on. I don’t share the idea that say the Greeks should have the same social level that the Germans do. Of course, if they will develop their economy to Germany’s level then it is ok.
Balancing will do nothing good, because richer, hardworking nations will not be happy when their revenues will be sent not on resolving domestic issues but to somebody else while the poorer nations will be complaining that the more powerful nations treat them unfairly. The nations in the Soviet Union, by the way, experienced this stance.