Are players capable of learning what the Scratch does beforehand if they lack any outside forces to learn so, or is the intended purpose of the scratch to wipe out absolutely everything including the players and the beta kids just got lucky?

I’m pretty sure by the mere fact you need Echidna’s Quills or something equally as hard to even pierce the Scratch Construct, that you would get informed if not by NPCs of the Land and Lore of the Battlefield, just by like. Prospit Clouds or the Denizens themselves. Also remember that in SBURB there’s hardly any source of ‘Outside Information’, the Horrorterrors are pretty much the Dersite Equivalent of Dream Clouds, and given they were fated to meet together and end up in a weird Chimeric Session, even the Trolls interacting with the Kids is kind of not that ‘outside’ of the game.