Was Angela Merkel’s role decisive for accepting migrants in Germany in 2015?

Yes. But at the same time the EU envisaged a mandatory quota system that would force EU members to accept refugees according to a formula based on population, GDP and so on. Many EU members rejected this imposition.

Interesting article here from 2016 http://www.telegraph.co.uk/news/worldnews/europe/germany/angela-merkel/12193876/Angela-Merkels-historic-error-on-immigration.html

A few months later - see some consequences of this here when the European Court of Justice ruled that member states would be forced to accept refugees http://www.telegraph.co.uk/news/2017/09/06/eu-court-rejects-refugee-quota-challenge-hungary-slovakia/

In any case, with open borders, even if a refugee was forcibly (?) sent to Poland, say, there would be no guarantee that the refugee would stay there; instead he may just move on to where he wanted to go in the first place.

The argument within the EU continues and may lead to a split – see this very recent article http://www.telegraph.co.uk/news/2018/01/04/eu-prides-unity-brexit-immigration-tearing-apart/

A more liberal viewpoint from 2015 is here… https://www.vox.com/2015/9/9/9290985/refugee-crisis-europe-syrian

And a German perspective here… http://www.spiegel.de/international/europe/how-eu-promises-to-introduce-refugee-quotas-failed-a-1040226.html

What a complex subject with so many aspects – humanitarian, political, economic, legal, national, international, social… Sometimes I’m glad I’m not a Prime Minister or President.