Is "Der Produzent" the same as "Der Hersteller"?
"Der Produzent" is often used in the media business, while someone (often the company) making physical goods is usually called "Hersteller". But it's not wrong to call them "Produzent" either.
Is there a distinction between "has not tested" and "did not test." In english, i believe "has not" implies there is still time to test (has not tested yet); whereas "did not test" suggests the opportunity has passed.
You are correct
Yes, just more formal and less used (in my region) "der Produzent".
In American English (is that an oxymoron?) the following would be just as accurate as your translation: the producer did not have the metal tested