re: If all the Native Americans had united, could they have stopped the White Man?

We would never have had a point of constant warfare until much later in time when weaponry advanced something considerable. Guns don't mean shite when you're outnumbered a hundred to one or more, have no clue about the land, and cannot transport enough supplies for long enough periods. Many explorations were privately funded.

The earliest explorers spoke of hugely populated coast lines with fires producing so much smoke that the water looked like it had fog on it. They spoke of the natives with reverence; powerful men who would crush them physically.

Nobody could have come here and fought the natives to a clean victory without a huge sum of them being wiped out by plague first. The Vikings tried, they got their asses kicked. There was a massive trading city in St. Louis as far back as 1200. The interior of the US was well traveled and Indians would frick you up if they felt you were not to be trusted.