The archetypes (except for the Hero) are actually not that important. Archetypes like the Trickster, Threshold Guardian etc. are kinda window dressing on the main idea of a Guy Going Somewhere. The specific characteristics of the Hero are also fairly useless. It is better to think of him (the gender of the archetype has some believable justification... it is more male than female) as a generic variable x, undergoing a transformation arc x(t), the journey.

The collective unconsciousness angle is not, in my view, very credible or useful in the form Campbell (and Jung) conceived and developed it. They were getting at something important, but I don't think they actually got it.

Throwing out the bathwater, the baby in the model is the character and narrative arc it suggests, x and x(t). The monomyth is very much a character-driven arc. The plot devices may look external (like "trials" and "inner sanctum") but derive naturally from the character arc.

My hypothesis is that the basic narrative/character arc combines elements of the stress response arc (flight/fight -- increased and then decreased tension in the face of danger as you react and then successfully overcome the danger) and the learning process (the model known as the Yerkes Dodson law, as well as the hypothesis/verification process of validating new knowledge).

I can't go into full details here, but I devote a chapter to it in my book, Tempo (http://tempobook.com)