Along with Heathenry I have over the years picked up some Taoist and Hermetic philosophy as well which posit human beings as miniatures of the Universe. “That which is Above is as that which is below, that which is Below is as that which is Above” Hard Science also, to a degree, backs this idea (“We are star stuff that has become aware of itself” Carl Sagan) I stongly feel there is alot of truth in this.
So, given the above statement, if the Gods are “just in our heads” does that not imply their existence outside ourselves (As Below so Above)? Or if they are outside ourselves on a greater level of being, surely they are also within us (As above so Below) ?
As for the syncretism, I think history and linguistics have already demonstated that at least some of the Gods have deep roots in ancient cultures which have since diverged, bringing their Gods with them, the names changing to suit new languages e.g. Tyr-Zues-Jupiter. I also think of the similarities between the names of Donar, Indra and Andar. Also look at how much–mythologically–they have in common with Perun!

I agree, archetypes are hugely important for any humans. Particularly these days when mythology (meaning) is so badly confused in many religious circles with rational reality (science).

Good to see a healthy degree of uncertainty. Nice post.