If (and wow that is a big if) the earth is flat then
1: who benefits from us poor deluded folk believing it is round, why have we been duped?
2: When traveling from A to B on the earth's surface (or above it in a plane) why is a great circle the shortest path (with a rhumb line as a near approximation)? if the earth were flat then it would be a straight line
3: is it only the earth which is flat or is the sun flat too? What about all the other planets?
There's 3 questions for starters I could come up with many more ........