Space agencies have already admitted to fabricating images and altering the images released to the public.
Well, firstly, let's be careful about the word "fabricating". Space agencies do produce images which are intended to show what future missions may look like, or they're imaginings of what, say, exoplanets may look like. But, as you say, they're always clearly labelled as such so they're not evidence of anything underhand going on. They're not claiming they're real pictures of something.
As for altering yes, they admit they're producing composite images, they admit they're enhancing images. As I said, every time you take a panorama or use your phone's photo editing tools you are altering an image. That doesn't mean the image is fake.
The flat earth was the accepted model before the globe earth model.
Right. Like there being 4 elements - earth, water, fire, air - was an accepted model before we understood about real elements. Like Newton's model of gravity was the accepted model before Einstein. As we, as a species, have understood more about nature newer models have always replaced older ones. The globe earth model replaced the flat earth one - that happened thousands of years ago. The heliocentric model replaced the geocentric one. That happened centuries ago. But the point is when that happens the newer model replaces the older one because it's better - it makes predications, it fits better with what we observe.