No, Stripe forms a C Corporation which separates the company's income from your own (as opposed to an S Corporation or LLC), which will prevent Germany's Welteinkommenprinzip from having an effect on your German tax return.
I believe that is correct. However as he is operating the company from Germany, he would be required to register the company as operating a branch (Zweigniederlassung) in Germany.
The company would then be liable for German taxes on income generated from Germany. German Payroll taxes would also be required if they employ people within Germany. I am unsure if the Branch Office is also required to pay IHK fees but that may also be the case.
Yes, it is separate. However, Ort der tatsächlichen Geschäftsleitung is Germany. That means you should pay taxes in Germany (if you do is another mattter, but if you don't and the Finanzamt finds out, you are in for a rough ride).
