I don't think its a matter of Sin Eaters only existing on the first and then Amdapor copies them. I assume Sin Eaters are a natural phenomenon when anything gets too much light aether as are Voidsent when they lost all their aether. I interpreted it as Ampdapor looked into their magic ball, seen creatures of pure light aether and copied them. At some point afterward the flood happened from mass light aether and the rest is history. My question would be, are the Lightwardens always the same or were they particularly strong vitims caught by the flood (or in Titanias sake, something more powerful that slayed the previous)