Making conclusions from this continuous universe model, such as the sun would never set on a plane, relies on many axioms of the continuous universe model being true.
Actually, all it relies on is some common sense.
How do I see something? Photons hit my eye. If that is from a light source then the photons leave the light source and travel to my eye along a straight path.
Or, the light travels from the light source, reflects off an object and travels from that in a straight line into my eye.
Either way if there is an unobstructed line of sight in between me and the light source/object then I will be able to see it. The only limitations to that are:
1) My visual acuity
2) Atmospheric conditions
So on a plane:
Clear line of sight, I can see the whole person.
On a curve:
The bottom of the person (or ship or sun) is occluded behind the hill so I can only see the top part.
And I see you have still ignored my thread about long shadows at sunset which prove conclusively that either
1) The sun is physically low in the sky or
2) The light is bending so it appears to be.
There are no other options but feel free to do some experiments and show how you can cast a long shadow of an object on the ground without the light source being physically close to the ground.