Could a post-scratch time player with enough mastery of their abilities be able to go back to before the scratch happened or send items back to prevent doomed timelines?

Nope, because the Scratch isn’t JUST Time. It’s Time and Space. Trying to go back before the Scratch through Time Travel would be just as futile as trying to go back to the Troll Universe by going back in time in the Human Universe. The Scratch erases the Pre-Scratch Iteration of the Universe and everything that’s within, to replace it with a new one. You would need dedicated machinery to reach a point prior to the Scratch that’s contemporary to yours- which there actually is in the Frog Temple. But God-Tier-wise? You would need both Time and Space to make it work.