Barry Fell, as I recall, read almost any kind of engraving as if it were ogham, spelling Phoenician? But ogham may be a post-Roman Irish adaptation of the Latin alphabet to spell Gaellic?
If the "Celtic crosses" on the glacial re-exposed pavement near Kimberley are “much older and possibly ancient Egyptian” and identical to Native American medicine wheel crosses, and navigational instruments and building construction squares to boot, then your rhetoric makes the Khoe and Kora speakers into neo-Egyptian navigators and architects.
These half-San half central African people were here before the Old Kingdom, with their full cultural repertoire, including representational and abstract art. Similar to abstracts in Ice Age art (see my paper on abstract geometrics in Expression). No need to be taught geometry by envoys or migrations from Egypt. Ditto Amerindians.
Your Hopi have some African and Asian genes, not implying they had Egyptian art, ritual, myth, etc.
You cite the late Brenda Sullivan, whom I knew. Your quote from her does not support diffusion. She was a diffusionist, but following Fell she tended to ‘read’ abstract shapes as carrying meanings defined by head office. Ironically she used more rhetoric than any lexicon.
If you see abstracts as degraded symbols, far from head office, you are falling into the same trap as De Santillana and Von Deschend, who read Icelandic myth as degraded astronomy, instead of the standard set of inspired, universal cosmology elements in myth.
Rather study Vinca, Aztec, etc sets of signs, to see how the standard set allows a thick layers of conscious options. Shapes do not diffuse. Sets of allocations do. Egyptian iconography is very well studied, and there is no trace of it on the rocks of Southern Africa. Or the Americas, except for a few made by a few lost Egyptians. They are as remarkable as the Egyptian-styled engravings in Australia. Which did not become diffused into any Australian art or icons or songlines. Some genes probably did, as they did in the shipwreck area of South Africa’s south coast, where some tribal leaders have lighter skins. Where diffusion of Portuguese technology and agriculture and Christian religion occurred, these are well known, and did not degrade beyond recognition.
Most adopters of imported culture, as in the Greek and Roman world, did not get the genes along with it. Mongols spread mainly genes, without spreading much culture. Perhaps you should isolate your study of genetic diffusion from your study of elements of cultural diffusion.
Isolated elements are diffused, not full repertoires. Thus part of the evidence that all people have full repertoires. In the same structure as that of the conquerers.
Structuralism is based on the philosophical concept of archetype. No amount of philosophy could erase that. Levi-Strauss used a model abstract enough to trace aspects of perception, but too abstract to model behaviour. French philosophy may have invalidated that particular model.