I agree with Ahmadi and Craig. I think that modifiers that answer questions of the type "Where?" or "When?" are always adverbs. In your sentence, "back" (= "backwards") answers a "where?" question (actually, a "whither?" question) so this makes it an adverb.

no, here "back" isn't an adjective, it's an adverb

In the sentence "Ron took a step back," the word "back" is an adverb that modifies the verb "took." It indicates the direction in which Ron took the step. It doesn't directly modify "a step"; instead, it provides additional information about the action of taking a step.

No. In the sentence, "Back" refers to the direction that Ron moved in. He was going forward...then took one step backwards. This sentence can be quite literal or could also be figurative in that Ron, had second thoughts about something.