Bluetooth Remote Controll for Voice input on Nexus 5
I found a solution to the initial problem. It seems that this is a common bug and in the software since at least Anroid 4.2. There is an Application in the Play Store that does nothing but reacting to the Bluetooth button and starting the voice recognition:

Voice Command

When I push the Button, I have to select what app to start, I chose "Voice Command" after that I am asked again and chose "Google Now" and then it starts directly in the Voice Recognition mode.

I don't know yet if it gets the commands over bluetooth or over the built in mic but it stops the music and starts the recognition.

This works at least with My phone (Nexus 5, 32GB Android 4.4.2 All settings to german and my Car Radio (JVC...))