Why do not the sun and moon change their apparent size as they come overhead, then receed in the distance?
Perhaps they do. People even explain size differences with "light refraction".
Why does it not take 2-3 times longer to fly accross the "southern hemisphere" then the "northern hemisphere"?
I don't even know if it does. How do you know it does?
Also, "hemisphere" implies a globe. Surely you're not looking for the globe theory answer?
Why can I see from a mountain islands I can't see from the beach?
To answer your question simply, a flat earth does not have to erase the way basic 3-dimensional perception works. If you are elevated higher, you can see further/different angles of objects.
As for view perception, the way it seems to me, what is below you climbs up, appearing smaller and smaller the further away it is, until it meets what seems to be in the middle which seems to make up the horizon. What is above you climbs down, appearing smaller and smaller the further away it is, until it meets in the middle as well. So whatever is closer to the horizon (basically the end of what you can see, which sits at your eye level?) is harder to distinguish, as it gets smaller and smaller. Do you disagree? If so, then on what exactly?
1. When you stand on the mountain, you are higher, which seems to raise the horizon. Thus the islands wouldn't necessarily be lost in the horizon anymore. By climbing the mountain, you are pulling the islands downward within your perception (because as you climb, the horizon seems to climb with you - but the islands remain where they are). You might see the islands better from the mountain because you raised the horizon above the islands, so they are no longer lost in the horizon.
2. The higher you are above the islands, the more surface area of the islands you might see. Put a pancake on a table. Kneel so the pancake is at eye level. Then stand up so you are looking downward at the pancake. You should see more pancake from this higher point of view.
However, it seems the globe earth theory is the one that has a bigger problem with visuals. People are claiming (apparently with video proof) that we can see too far. Way, way too far. Structures that should be far hidden by the curvature of the earth are apparently visible, precisely as one would expect on a flat earth.
These are things that have to do with my personal observations.
Much love!
That's fair. My personal observations are far too limited for me to conclude the shape of earth, haha