Ehhh, not quite? The heart of the sepulcher, which is where the Heart of Eternity is located, is basically at the very core of the Universes ether (You can see the cosmic influences in their most basic state from there VIA the skybox). However, based off the implications of the lore, there are seemingly other cosmic hearts akin to the Heart of Eternity, which serve as core engines within their respective cosmic inner sanctum (and are likely located elsewhere within the universal ether core).
Zereth Mortis, and other cosmic Zereths aren't really located in the cosmic domains themselves. They have their own specific location that starts off as "being linked to the domain itself" (Which is why Zovaal was able to claim a piece of it during patch 9.1), and as you go further in there, the realm gets further separated from the domain, all the way until you reach the core of the universal ether, where the cosmic engines are completely separated from the domains.
- - - Updated - - -
Unless it gets retconned, SL lore will most likely extend itself into the Worldsoul Saga.