Re: " geostationary satellite's key missions "
Given the Earth is effectively all rounded corners flat earthers can be either correct or entirely wrong it all depends on whether you think gravity is holding earth correctly.
The only way I can see flat earth theory being debunked if we build a massive pool cue and a chalk the size of the moon and attempt to add spin to the earth.
I think all theories are possible, it just depends in which dimension you currently reside.
In my dimension...5...we mock people that think the earth is spherical as it was conclusively proven that the earth is a zorgflaargh when a certain chap went round, through, parallel to, adjacent to and none of the above (simplified for three dimensional perception) in his vehicle that was able to move and remain stationary throughout the journey he simultaneously undertook and didn't bother with but also half arsed his way through.
Then again what do I know, I come from a dimension where the 5th axis is inclination where we can measure things based on whether its worth bothering using a tape mehsure calibrated into units of meh-gaherz.