Amazonian is the proper adjective for Amazon.
Proper adjectives are similar to proper nouns. Amazon would become Amazonian.
The proper adjective for Amazon is "Amazonian."
The proper adjective form for Amazon is Amazonian, such as the Amazonian rain forest.
Large and varied are both adjectives. South would be an adjective if it were not part of a proper noun. Amazon is likely a noun adjunct here, but Amazonian would be a proper adjective.
The proper adjective for Portuguese is "Portuguese."
The proper adjective for southwest is "southwestern."
The proper adjective for Haiti is Haitian.
Vietnamese is the proper adjective for Vietnam.
The proper adjective for Siam is Siamese.
The proper adjective for George is Georgian.
The proper adjective for Georgia is Georgian.
The proper adjective for Mayan is "Mayan."
Freudian is the proper adjective for Freud.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.