“They’re the post-scratch versions of the Dancestors” that always confused me, how are those guys able to become who the ancestors turned out to be? Either I’m missing something big or I’m not well informed on how the scratch works

They… Didn’t? They’re Post-Scratch. They’re an entirely separate set of characters from them- Babies get sent through the portals. Scratch changes when these babies arrive to Alternia. In one reality, they arrived later and were the Players. In another reality, they arrived earlier and were the Parents.