Okay, I'll bite.
No, I'm the one doing the "biting" here. You're just trying to waste people's time, as demonstrated in the quote below:
(is that even a real word?)
Depends on what you consider "real", really. A quick Google search suggests that it appears in some dictionaries, although not the ones I'd normally use. If it makes you feel better, I'll use "opacity" in the rest of this post.
Just how does the intransparency of the atmoplane cause the existence of the horizon?
Let me try to explain this with a thought experiment (although you're welcome to try it out in real life). Picture an empty orange Tic Tac® box. We'll use the orange flavour to make the effect more apparent.

As you can see, the box is somewhat translucent, but not entirely transparent. It is, to an extent, opaque. This means that any light you see through it will be less luminous than it was at the point of incidence. Because the box is orange, this will be most noticeable for blue and cyan lights, but since perfectly transparent materials don't currently exist, it could also be measured for light of any colour. If you were to look through this box, the world around you would look considerably more orange, and overall somewhat darker.
Now, imagine two such boxes lined together. As you can hopefully imagine (or verify yourself if you feel like getting a bunch of Tic Tac®), looking through two boxes would make things seem even more orange, but also more dark.
Now, imagine many boxes. Like, plenty of boxes. Make sure your kids aren't looking, 'cause if they find out those dental bills are gonna be insane.

Try looking through all of those in a straight line. Can you see anything at the end? Chances are the answer is "no". If the answer is "yes", please add more boxes to the simulation until the desired effect has been observed.
To nobody's surprise, opaque materials are opaque. The atmolayer is opaque. Once light passes through enough of it, it eventually dims out completely.
The sun in FE is 3000 miles overhead. When it moves 5,196 miles away it will be twice as far from you (30-60-90 triangle, ignoring the curved flight path in FE for the moment).
You seem to assume that the Sun will be directly overhead at some point. Of course, it never comes anywhere close to that. It's almost as if you were trying to create your own FET just so that you can "debunk" it.
Almost.

But yes, if you ventured far into the lands beyond the Ice Wall, it's quite likely that you would observe this.