Sounds like your cousin's husband would be a kindred spirit to the old professor I studied under. His name was Edward Milligan and he had been adopted into the Dakota tribe. He published under the title Historic Migration of North American Indian Tribes and his work was largely pooh-poohed by the experts, many of them for allegedly superficial reasons like Prof. Milligan's imperfect grammar in writing.
His most memorable statement was squarely aimed against some of his critics who still contend all original Native American migration was over the Bering Strait land bridge. The Prof. pointed out that every other civilization built their greatest cities near where they first entered the continent, then more rudimentary dwellings as they moved further away. "Why," asked the professor, "should only America be the opposite?" The greatest evidence of civilization are concentrated between Peru and Central Mexico. Northern Mexico and the American southwest still have some fairly impressive but less developed evidence of civilization. But as you move into Northern California and up the Pacific Northwest to the Bering Straight, evidence of even rudimentary civilizations grow even more sparse. You can't dismiss it by climate alone because at least the southern portion of the Pacific Northwest has a climate far milder than the mountains of Peru where these ancient people thrived.
I’m going to look for that book/paper tomorrow-I’m very interested in reading it.
The trade routes of early people in part the Americas can be followed from the seacoasts in Mexico, too. You can visit the ruins of the port cities, then the ruins of the trading/distribution centers inland and leading north, with the places getting smaller and less refined as you go inland and farther away-sort of a western caravan route, except that the goods came over the sea from far places rather than overland from those places.
If they traded the same way as people did later on, the best stuff-exotic luxury goods and such-was probably offered first in the costal and other big market centers for maximum profit and already traded out by the time the traders reached what passed for the frontier in that age. They probably loaded their pack animals with hard-to-get simple treats like cloth made of something softer than rough wool, finer animal skins, better tools and inexpensive decorative items for the homesteaders. There was no paleoindian/preclassical equivalent of the Sears catalog back then, after all...