That they were able to get from Vienna to Luxembourg in one day for the summit meeting, doesn't seem possible.
The killer (and would-be multiple assassin) is named Lazar Kiss. They say he is part Hungarian, and Kiss is in fact a common Hungarian name. But it is pronounced "keesh", not "kiss", and the police in Vienna, the western capital of the Austro-Hungarian Empire, would have certainly known how to pronounce it.
The whole supposedly cunning plot hinges on the girl pickpocket Lily being able to lead the police pursuing her all the way to the upstairs room where the murdered body is. She is chased through the streets by the uniformed police officer, who is calling out for passers-by to stop her. Had just one of them done so, the whole plot would have fallen in a heap. Not so fiendishly cunning after all.