The Astral Keys and Mechanus are both core components to running the universe.
Mechanus has been called the "breaker box" or "control panel" of the planescape. Primus is able to manipulate the law of physics and makes sure everything is running the way it should. Artificers learn how to manipulate matter and build mechanisms from Mechanus and how it functions, the traditions kept there. Even in civil war, Primus does their best to keep the structure of reality in tact.
So where does the Astral Keys come in?
Where Mechanus is the breaker box, the Astral Keys are the electricity, the wiring, the switches. A book written at the start of the universe in a language only beings born in the era of its creation can read, the Astral Keys contain the codes to reality itself: the nephrons, every plane, and every person. When the nephrons were stolen, Teres was able to get his hands on it to produce his blade. However, the book follows no real laws but its own. It traveled across every plane, falling into the hands of many.
With the Astral Keys (and nephrons), Platysma plans on becoming the Astral Witch. A failsafe at the end of the book allowing someone of intense magical ability to make themselves a creature beyond gods, beyond eldritch beings. Beyond time and space. A creature who can recreate the entire world and reset it in case it is ever destroyed.
So while Mechanus tweaks and changes the present laws, the Astral Keys can flip everything on its head instantly, altering the state of the universe.