My lowly two cents worth ... is, that the ancients trekked in both directions across the top, entering/leaving present day North American around both sides of present day Hudson Bay.
Some time later, some ancients trekked east/west across present day Alaska.
The preceding groups were prone to be *both* nomadic and semi-agrarian hunter-gather-er.
Other ancients were trekking north/south between present day North American and present day Central America.
These groups were prone to be less nomadic than the above groups; and these groups were more agrarian, also building longer-lasting towns and cities - many constructed on mounds (these were “the mound builders”).
The notion that any of these peoples were innocent, peace-loving agents working for old EPA regimes in order to preserve the planet for posterity ... is wrong.
When more often, they were inclined to “rub out” their enemies, though saving some as slaves and adopting fewer numbers.
I suspect that the “mound builders” became physically, relatively weaker than the more nomadic-prone (willing to hunt over great distances), which is how the “mound builders” lost many struggles between the 14th - 16th centuries ... eventually being replaced almost completely by the 17th century.
Leaving the many clans, families, and tribes encountered by settlers, starting with the 17th century.
Something like that.
I agree with much of your scenario, although I do think that the most likely reason the mound builders were so depleted by the 1700s was due to disease brought from Europe. It is estimated to have wiped out up to 90 percent of the pre-Columbian population. As for the relative size of the nomadic hunter gatherers, I have read that they were not as large, and resided much further south, prior to the Spanish. After they got horses, they rode north for the buffalo hunts, ate a lot more meat, got more populous, and grew in average size and population. I have no doubt there was lots of internecine warfare as well. They were not angels, although some tribes were more warlike than others.