ABU, called my ex wife, (hated it but had to do it) she is a Verfassungsrichter in Karlsruhe (Constitutional Judge), which as you know is the highest court in Germany. She points out the following in the German Constitution

Freedom of religion in Germany is guaranteed by article 4 of the Grundgesetz (constitution) stating that "the freedom of religion, conscience and the freedom of confessing one's religious or philosophical beliefs are inviolable. Uninfringed religious practice is guaranteed." In addition, article 3 states: "No one may be prejudiced or favored because of his gender, his descent, his race, his language, his homeland and place of origin, his faith or his religious or political views."

She tells me she will look into the immigration paperwork to check out what you have said but she points out that NO ONE can stop you filling out Islam under the religion question as this is constitutional not permitted by the above Article 4.irrespective whether you are foreigner or German National

She also tells me that the law for head wear is what I stated although in recent years some German Authorities turn a blind eye to Muslims and there head scarfs as this is a tricky subject at the moment within Germany