1) It would be astounding if, in the year 2019, the author of the graffiti can't be identified from a surveillance camera. So we'll probably know all about him soon.
Islamists here tend to be of North African origin. German isn't their language of choice. So my guess is when they find the perpetrator, which probably they will, they will not find a proper Nazi.
Either way, I'd guess it won't be someone who has spent many years cultivating his Blutbewußtrein and lovingly studying the proper way to scrawl that word on shop windows.
6) The French government responded properly. *This* French government. What another would do is anyone's guess.
7) That's really all I have to add, I guess.