What part of "Astronomers have mapped out the entire southern celestial hemisphere" don't you understand? That's exactly what those surveys I posted have accomplished.
How can they have mapped out the entire southern celestial hemisphere if they haven't observed it from all possible locations? How can they be sure that it would appear the same everywhere?
Because from any good vantage point on the Earth, one can see half the celestial sphere, from the zenith (90 degrees altitude) to the horizon (0 degrees altitude) and all around (0-360 degrees azimuth, measured NESW from the meridian, which is an imaginary arc connecting the zenith to the north horizon). Given the time of day and latitude, one can convert altitude and azimuth to right ascension and declination. Because the celestial sphere rotates around the Earth, one can map out the same portion of the celestial sphere from anywhere at the same latitude. This is how the VLA, in NM, was able to survey the entire sky north of -40 degrees declination.
Because the shape of the Earth is a relatively simple one. It's an oblate spheroid. If you really can't see how straight light propagation with a round, rotating Earth, consistent with the existing laws of gravitation and electromagnetism, is simpler than a flat Earth with bendy light, which contradicts both gravitation and electromagnetism, then I'm not sure what to say to you.
Bendy light contradicts neither gravitation nor electromagnetism.
Bendy light doesn't contradict gravitation, but a flat Earth does. Bendy light does, however, contradict electromagnetism, which has no provision for the kind of dramatic bending required for a flat Earth to appear round. Only in GR do we observe bending of light, and even for things as massive as the Sun only a miniscule amount of bending is observed. Astronomers, however, can use a process called gravitational lensing to observe objects extremely far away. In this case the object bending the light has to be immensely massive, such as a galaxy cluster or a black hole.