1. Most scientists don't know exactly how gravity works. Gravity is a theory, regardless of one's view of earth and its shape. If the earth isn't a spinning ball in "outer space," then it stands to reason that objects don't simply drift away, provided they have enough buoyancy.
2. The "objects disappearing over the horizon" has been addressed by numerous YouTubers. If one has a telescope, it will manifest a boat or other object that has sailed past the "curvature." Such would not be possible if the earth were round because the curve would always obsure the view once something was far enough away. Moreover, not all objects descend from view as we move farther away; some actually ascend, depending upon topography. Remember, even a flat earth has mountains, valleys, and uneven terrain.
3. I agree with other posters that the sun is possibly a spotlight rather than a glowing orb. Again, if our world isn't what we were taught, it stands to reason that the sun, moon, and stars operate differently. Anyone who has visited a planetarium has seen that the sky can be projected differently depending on where one is viewing. The larger the scale, the easier it is to show different parts of our world different images of the sky.