This is partly correct, yes... but nevertheless you are not allowed to work inside the Country (Hint: Workpermit) while holding a DTV... that's what I said.
Right, but you know that you have it and what kind of stamp you expect... and since everyone should check the stamp before (!) leaving the counter... well... and so on... 😁
A passport in Germany is not a proof of residency, because there is no address in it. I know that you can have your complete address in your passport in your country. As a proof of residency we use the ID-Card... or the Passport together with a "certificate of residency". Since he doesn't have a valid residency in Germany he can't apply for this certificate.