IR refers to "internal rotation." IR is a basic anatomical body motion. If a kid can't do IR of the forearm, the kid can't open a door. So, yeah, they all know forearm IR. What they don't know is how IR is incorporated into the pitching motion.I know you were describing how naturally a body could move IR but that doesn't mean that's what's happening from the start.
Young kids usually start pitching using HE because that's the way Dad throws batting practice. Of course, Dad is trying to make the ball easy to hit. Softball pitchers are trying to make the ball hard to hit.
All high-level pitchers throw IR, including Fernandez, Scarborough, Garcia, Abbott, and on and on. I could list 100 pitchers. I've never seen any college pitcher use HE, but maybe there's a couple out there somewhere.As you commented eventually may change. But then again not all do.
If a kid comes in for instruction and wants to be "good," a PI should teach what the good pitchers do.
As to style variations: Unlike baseball rules, softball rules define the pitcher's arm slot and, to a large extent, the starting feet position.