Posthuman,
If you were to go to Antarctica and watch their summer solstice, and then go to the arctic and watch our summer solstice, which is incontrovertible proof of a spherical earth, they would still not believe you.
I doubt if the FENers themselves could be convinced to make the trip, but even if they could do you think it would change their mind?
I think what we are dealing with here is people who have an agenda to discredit Christians, not people who are so deluded as to really believe in a flat earth.
Truth doesn't matter to them, only their agenda. They are working for antiChrist, not truth. One can never reason with the devil. It is pointless to try.
If they were sincerely deluded, I think they would consider what you have presented, if it is possible to reason with a deluded person. But, because of their attitudes and responses, I am dubious that that is what we are dealing with here.
Just my opinion. I am open to being proven wrong.