Learn Dutch: Die amateur is geen professional meer. - That amateur is no longer a professional.

In Dutch, "not a" is always translated as geen. To say "no longer a", you use the combination geen ... meer, placing the noun between them. You cannot use niet with een in this context.

If the sentence were positive, would we say Die amateur is een professional?

You can, but Dutch typically drops the article for professions and roles. A native speaker would more likely say Die amateur is professional. However, in a negative sentence like this one, we must use geen as the negative article.

How would I say "He acts as a professional" to practice this lesson's distinction between als and zoals?

You would use als to indicate a true role: Hij gedraagt zich als professional. If you used zoals (Hij gedraagt zich zoals een professional), it would mean he acts like a professional, which compares him to one and implies he might actually still be an amateur.

Are the loanwords amateur and professional pronounced the English way in Dutch?

Professional is borrowed from English and sounds nearly identical. Amateur, however, is borrowed from French and keeps a French-sounding stress and vowel on the final syllable (ah-mah-TEUR), unlike the English pronunciation.