Walther, Rohm, J. P. Sauer and Sohn, Blaser, Max Ern, F. Henninghaus, HEYM AG are a few.
Right off hand, I can think of a couple: Sig Sauer, Walther, H&K
It is unlikely. The suffix -stein in last names generally indicates Jewish descent.
German for German Shepherd is 'Deutsch Schäferhund." It's also known as the Alsatian.
Yes, the name Marks IS a German name.