Hey, thank you for such a lenghty answer. I really appreciate your effort in making your point clear.
I see some interesting aspects in some of your argumentations. You said that, for example, we can prove that the theory of the round earth is valid by showing that the sun goes down, but this does not constitute a proof: in only shows that the empirical evidence is in accordance with the theory. As you say, more than one theory can predict a phaenomenon, but this does not mean that all those theories are sound.
I may come up with a very imaginative explanation for the sun movement: it is the mystical chariot of Apollo, traned by horses. This explains why the sun moves. But this is not a scientifically accurate description, and thus it should not be accepted as truth, not even an "alternative", or "equally valid" truth.
If your theory needs to acknowledge the existence of a god which, for some reason, feels the urge to ride a magic chariot through the sky... then you have a problem, because of weak inner coherence (the same tradition tells that Apollo does a lot of other stuff during the daytime... so who drives the chariot then?), has no other empirical evidence to support itself, and so on.
If I didn't misunderstand, your support for Flat-Earth is mostly a reaction to a perceived "tyranny" of standard science (it may also be a sincere personal belief but I'll ignore this aspect). While this position is understandable and even worth of praise for the innovative spirit that it carries on, it can lead us to ignore legitimate evidence of the currently accepted theories, or to overvalue or misinterpret any alleged evidence of the contrary. The vice-versa is also true: a blind faith in standard theories is equally harmful because of its detrimental impact on innovation and discovery. A good theory should be falsifiable, and an earnest theorist should want its theory to be falsified as soon as possible.
Your discredit of some common accepted scientific concepts, for example, seems a bit hasty to me. Gravity can be easily observed in laboratory: the Cavendish experiment has been repeated countless times since the late 18th century, and can be recreated with very mundane equipment. Magnetic declination is simply a definition of an angle, given by the difference between the magnetic and geographic poles: as such, it is still perfectly valid even if we accept flat-earth.
Anyway, I still can not agree with the assertion that standard science rejects other theories without proving them wrong. It is the job of the new theory to prove itself better, and in fact, if a new theory shows to be more apt to describe reality, it is generally accepted. The issue is that, as far as I can tell, Flat-Earth theories may have an hard time substituting accepted models: a good FE theory should prove itself "better" than the pre-existing theory. What validates a theory is a mix of several factors: inner coherence, accordance with empirical data, and so on. If two theories are more or less equally able to explain the world, generally the simpler one is preferred: the one that make less arbitrary assumptions, the one with the simplest and more elegant math, etc. An instance of Occam's razor, if you wish.
Now, why should I prefer Flat-Earth over Round-Earth? I subscribed to this forum to find out, with a very open mindset, but I am a bit disappointed to see that there's so little agreement among flat earthers, even on crucial aspects like the orbit of the sun, and the issue is dismissed by saying "both models are valid". This sounds very unscientific to me.
Please forgive my frankness, and thank you again for your contribution. I am sincerely grateful for the time you spent ansering me.