In the introduction to Joseph Campbell's "The Hero with a Thousand Faces", he cites sources that state the Minoans made their way as far south as Nigeria. Is there substantial archaeological or historiographic evidence for these ideas or is it just conjecture?

Ok, lets start off with a rule of thumb. Scholarship of Africa from the 1920s is extremely out of date. As a discipline, African history was extremely young in the 1910s and 1920s, and there was a great deal of pseudohistorical crap being published back then. Ideas of Lost Colonies or "lost races" and extreme diffusion of "hamitic" or middle eastern peoples were common both in Academic histories and in pulp fictions. Most of the time, if you encounter an extraordinary claim about African history, and the source is from the 1920s, you are pretty safe in not believing it.

Anyway, no, nobody takes Frobenius' theory of Minoan contact seriously anymore. Frobenius' earliest claims point to the terracotta pottery of Ile-Ife and claim that such artistic mastery must be due to Greek (Minoan) influence. Now it is widely accepted that the Ile-Ife terracottas represent a local development.

For some flavor, here is an article from 1974 which argues essentially "yeah, Frobenius' theories are crap, but his firsthand observations of Yoruba religion from the 1910s are still worth examining, if you keep in mind his bias"

here is a second article about the study of Yoruba religion, placing Frobenius in context with earlier and contemporary Egyptologists and missionaries who came up with all sorts of "origins" for Yoruba religious practicw.