Knowing where the viewport is relative to the document is pretty valuable; It means that AMP can defer loading potentially sluggish content until the viewport lands on a particular position.
Sounds like a premature optimization to me. The websites I tried did not have such requirements but the scrolling was just as janky nonetheless.
It's not premature optimization. It's optimization for ad revenue; they can charge more if they can prove an advertisement is visible on screen.
For surveillance and user tracking.
Even if this would be correct (which it is not), it would only require to observe, not influence scrolling.
Maybe not in version 1. Think bigger, you'll get that promotion!
This is just CSS which exists for unrelated reasons.