Perhaps my understanding of the Equivalence Principle is incorrect as well, which is eminently possible (and I know there are weak, Einsteinian, and strong versions which complicate things further!). I thought it was the inability to distinguish between being stood on the surface of a massive body, and being inside an accelerated frame of reference - the key word there being "accelerated".
I do get what you are saying, and I considered the whole "all moving as one thing", but like I said, the key word is "accelerated". A similar analogy could be a ball on the floor of a train. At a constant velocity, the ball, the train, me, the whole system is traveling at the same rate so everything just stays in the same relative position, doesn't matter what the velocity is. We can't feel constant motion. As the train accelerates we are all subject to the same acceleration, I feel the effect of that acceleration but I'm fixed in my seat so I don't move - I'm essentially part of the train. The ball is not fixed to the floor, it's free to move, and so relative to my direction of travel it appears to roll backwards away from me (falls, in effect).
Is that not the same as your rollercoaster example? Under UA, everything - the Earth, the rollercoaster, the rollercoaster car, the people strapped in the car are all moving upwards as one system. If the Earth were just moving up at a constant velocity, in the absence of any other force your last statement would be correct - there would no reason for the riders at the top of the hill to change their position relative to it. However, if the Earth were accelerating and changing its velocity over time, the car is free to move relative to the track which is fixed to the Earth. Riders strapped into the car at the top would now effectively roll downhill. It's the acceleration of the Earth upwards that gives rise to the apparent force acting on the car, i.e. gravity. This is why they are equivalent, and this is why flat Earth theory uses it in its sub-theory of Universal Acceleration...is my understanding anyway!
I dunno' mate, I might have it all wrong, it's really interesting stuff nevertheless. It's certainly easier to wrap your head around an accelerating body giving rise to the "effect" of gravity I'll give them that!