The first thing you do in Zereth Mortis is warn them about the Jailer & the Maw...they have no idea what you're talking about.
So I say again, wanting to destroy the design makes you outside the grand design. The Automata know about everything, except the Mawsworn. Stating the First Ones "put the waystone there" being proof that everything that happened in shadowlands was intended by the First ones is just....dumb. We have no reason to think they put a waystone there. Rather, the more likely explanation is that the location the Eternal Ones turned into the Maw was part of that Forbidden Area (Korthia) no one was ever supposed to go, including the Eternal ones, and predates the Cosmic design entirely.
If the First Ones predicted the events of Shadowlands they simply could have told The Primus not to make Frostmorne. Or stopped Denathrius from making the Dreadlords. Or fixed Zovaal's brain. Or given the Arbiter higher bandwidth. Or Aborted Sargeras. Or countless other things. All would have ruined the Jailer's scheme. This idea that it was supposed to happen isn't established anywhere in the story & somehow makes the storyline even dumber than it already is.