It shouldn't be hard to prove the shape of something. If we had satellites in Venus before we even went to the moon. Im sure we have one far enough to take a picture of the whole Earth. The fact we don't, is funny.
You think they had cameras on those space probes that they could control from back on earth and point it exactly at the earth? And the cameras were high enough quality back then to actually get a viewable image of earth? Hell no.
You ever seen an old photo from the 60's? You could barely see somebody standing 10 yards away, how do you think a photo from Venus to Earth is gonna look?
Your other argument about how old civilizations believed the earth is flat, as proof that the earth is flat, is beyond ridiculous. Our knowledge advances and evolves in time. These people didn't know what electricity, computers, airplanes, etc. were, yet they are more knowledgable than us on whether the earth is flat or a sphere? Come on man, that's like the worst argument of them all. By that logic, smoking cigarettes is healthy because people used to believe it was.