The fact that any small portion of the Earth appears to be flat is not proof that it is flat. In both RET and FET, the Earth is considered to be huge, so in both theories it is perfectly logical that any small portion of the Earth will appear to be flat from close up. I have shown that even under perfect conditions, the visible curve of the Earth according to RET would be less than 0.5°. Imperceptible for a human. Considering that the Earth's surface is not perfectly even adds to that. So, when you say you look out of the window and the Earth appears to be flat, no matter where you are on Earth, you cannot be sure that the entire Earth is truly flat because it is not the only possible explanation for what you see.
As for magical UA vs magical gravity, since both seem to be equally likely but they contradict each other, neither can be considered to be true, therefore both must default to false until proven otherwise.
As has been stated in numerous threads, the assumption that there is a conspiracy is based on the assumption that the Earth is flat and vice versa. Since there is no proof for either, both default to false. Yes, the fake image argument is connected to this. I can merge them if you like.
As for the sun and moon, their size and distance is pure conjecture, created to fit into FET. Thus unless the Earth is proven to be flat, the assumptions made on the size and distance of the sun, moon, stars and other planets are completely worthless.
As for sustained space flight being impossible, yes it seems that this point is disputed even amongst FE proponents. Still it is an unverified claim in any case.