Apparently, the Paiute did - caught up with what was believe the be the last ones troubling them in a cave. Set a big fire at the cave entrance, killed the red giants that came out, while the others died inside. They had plagued the Paiute for decades, but they finally seem to have wiped out the local population of red-haired giants.
So that shoots down your first postulation - and then we have other narratives of Native Americans, including Inuit of them killing these things that were tormenting them.
One doesn't take toddlers hunting - at least none of the guys I've ever hunted with did. And if 10% of the narratives of folks getting the crap scared out of them - including kids at schools - are true, your next postulation falls apart.
I see you're big on practicing. Practicing this, practicing that.
Each spring around here, the birds lay eggs, and the eggs hatch and there are baby birds in the nest. I must get real busy or am not paying attention, but somehow, year after year, I miss these young birds going through flight school. Somehow I miss all the instruction, and all the practice. But then one day, they must graduate from flight school - as they're just gone.
Same with mountain goats. Those little bitty mountain goats are jumping around on a few square inches of rock like they've been doing it all their lives - and somehow, we seem to not be able to film or otherwise document their leaping training, their jumping skill training, and their escape tactics. How can we miss all that?
Oh. That's right. There are lots of reports of these things barely avoiding traffic on a highway - sometimes getting hit! And sometimes getting shot while snooping around a house.
Your postulations/questions ignore the fact that many of the projections you suggest - occur.
Here's the deal. Nature has her ways.