High Hrothgar and the Leaper King, are the head(s) of the world, for their place on the Throat of the World? Let's say Alduin is the Atmoran World Serpent, the name of his Yokudan counterpart, whose throat are we climbing?

The world.

the powers of Ada-mantia granted the Mundus a special kind of divinity, which is called NIRN, the consequence of variable fate.

So I guess I'm asking, is the Hrothgar the junction of many heads, or multiple fates?

The junction of generations, I'd wager, but of the Nordic heads only.

Edit: Hjalti climbed the 7000 "generations," and he came to be Ysmir. Now there's a new dragonborn, I'm curious to see which of the many heads of Talos he'll become.