after all the opening of the borders in 2015 wasn't lawful either
False. Not closing the border was completely legal under German law.
EU and it institutions and laws have nothing to do
The EU is an important amplifier of the political independence of its' member states, particulary the smaller, Eastern countries. Being inside the EU increases the political weight of each country while negotiating with Russia.
The EU is also becoming more active in boosting the military cooperation between member states with e.g. PESCO. Which is a good thing, considering the forseeable demise of NATO by American hands.
just another straw man argument.
A basic geopolitocal observation is the farthest from any straw man. Countries like Poland can use any alliance that increases their weight and independence from Russia. That is a fact.