A proper adjective for "Amazon" is "Amazonian." This term is often used to describe things related to the Amazon rainforest, such as its biodiversity, culture, or geography. For example, one might refer to "Amazonian wildlife" or "Amazonian tribes."
Amazonian is the proper adjective for Amazon.
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 southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
Vietnamese is the proper adjective for Vietnam.
The proper adjective for George is Georgian.
The proper adjective for Siam is Siamese.
Mayan is itself a proper adjective.
Freudian is the proper adjective for Freud.
The proper adjective for Georgia is Georgian.