“It is a common misconception that the sun would light the entire earth's surface at the same time.”
If the Earth were flat, it most certainly would light the entire surface at the same time.
You are suggesting that the sun will only light the portion of the Earth that it is over at a given time, but not other areas of the same flat surface that are father away (similar to how a flashlight only illumines a narrow circular area when aimed at the ground at close distance).
This is demonstrably false, since the sun begins to light the each location of the earth from the moment those at the location see it to rise in the east. It doesn’t only begin to light the various locations as it passes by overhead. Additionally, the fact that everyone on Earth sees the sun rise and sees the sun set, proves that the sun is visible to all from moment it rises to the time it sets.
"I'm constantly wondering, checking, experimenting, and asking questions, because the world isn't just for rocket scientists. Its for me, too."
Here is an experiment for you to try tonight. Go into a room with a table (flat surface), turn out the lights, put a small portable light (such as a candle) below the surface of the edge of the table, slowly lift it above surface level, and see if it lights the entire surface at once. Without ruining the experiment, I can tell you that it will. The surface of the table closest to the light will illuminate at the same time as the surface farthest away. If the Earth were flat, the same thing would happen when the sun rose in the East.
“My friend flew late afternoon from the East coast, to the West coast. For hours, the plane cruised at one altitude, while it chased the sun. The sun never set--for the plane. But down on earth in DC for instance, people saw the sun set as it moved out of their area at 7:00pm.”
That is exactly what we would expect with a global Earth. If the plane took off 2 hours before sunset time in DC, and followed the curvature of the Earth for 4 hours on its way to California, the plane would remain in a lit area until it landed three time zones westward. The sun would then set 1 hour after the plane landed – three hours after it set in DC.
The example you used refutes your position: If the Earth were flat, the sun, which was above the horizon in California when your friend landed, would still be above the horizon in DC. The horizon would be the same for everyone, and therefore the sun would set at the same time for everyone.
“the plane … never changed altitude and followed a straight level line for 3000 miles. How can a plane follow a level trajectory for 3000 miles and not fly off into space?”
Very simple. The plane followed the gradual curvature of the Earth from a height of 30,000 feet above the surface.
“The sun is not big enough to light the whole earth, but traveling at about 1000 mph, it gradually lights an area for 8-10 hours (up to 12) as it comes and goes to carry its light from East to West”
Again, this is demonstrably false. We know that the sun is visible from the moment it rises to the moment it sets. If you doubt that, watch it go down tonight, and wake up early and watch it rise tomorrow. We also know that the sun is big enough light each area from the moment those at the location see it rise, until the moment those at the location see it set. It does not lighten a particular location only when it passes by overhead.
“There are several videos on Youtube explaining how the sun works with perspective for people on the ground. You may need to watch a few of them to get an idea because its difficult to conceptualize what someone is saying until you see the experiments.”
I’ve watched some of these videos and they were absurd. All you need to know is that, if the Earth were flat, the sun would rise and set at the same time for everyone, and there would never be a location on the entire flat surface where the sun would not be visible, when it was high in the sky at another location. These points are irrefutable and no Youtube video can explain it away.