SPOILERS I have several questions

[Erin1491]

Spoiler warning this post contains info about the final two dawntrail zones.

First off, so we know in solution nine that they extract the souls of the dead and separate the memories from it. The memories are loaded to a terminal in living memory, while the soul itself is processed and becomes a soul cell. Is it ever explained how these would return to the aetherial sea? Can memories return to the sea without a soul?

Second. How is Otis able to be an endless on both solution nine and living memory. He volunteered to be experimented on and became one of the early versions of an endless correct? Which is why he inhabits an older model body. He sacrifices himself in the battle for solution nine. The queen implies that's the end of the road for him. Yet we still meet his memories at yesterland in living memory. How is this possible?

[RavLandslide]

Second. How is Otis able to be an endless on both solution nine and living memory. He volunteered to be experimented on and became one of the early versions of an endless correct? Which is why he inhabits an older model body. He sacrifices himself in the battle for solution nine. The queen implies that's the end of the road for him. Yet we still meet his memories at yesterland in living memory. How is this possible?

Otis offered himself up to be experimented on, not specifically to become an Endless but just general soul stuff. From these experiments both his robot self, with his actual soul, and his Endless, which are merely his extracted memories made form, were born. His robot self is also not considered an Endless, because it still contains his actual soul.
Sphene saying it was the end of the road for him was about the true Otis. The Endless Otis would still go on, but would not gain true Otis's memories from after the experiments.

As for the first question, I have no clue and am very curious too.

[Cleretic]

First off, so we know in solution nine that they extract the souls of the dead and separate the memories from it. The memories are loaded to a terminal in living memory, while the soul itself is processed and becomes a soul cell. Is it ever explained how these would return to the aetherial sea? Can memories return to the sea without a soul?

Answer here is unclear, but the Arcadion suggests that once a soul is properly set free of the Everkeep's cycle, it's gonna go back to the Aetherial Sea.

Question there is when the soul is actually freed from that cycle.

[Layte_Aeon]

Firstly memories are themselves a type of aether, the immaterial sort as per Mikoto's explanation in the Ivalice raids. Such aether, without a vessel, naturally disperses back into the aethereal sea, where they are broken down into their constituent bits. The actual content of the memories isn't important to the aethereal sea.

As for Otis, he was part of the experiments to create Endless true, but something you should remember is that memories can be copied. That's part of how soulstones work. They contain the memories of techniques, but the people who made those techniques still kept the memories with themselves as well. The Meso Terminal is, in essence, a very big, complex soulstone, containing the memories of those wearing regulators.

[Zero-ELEC]

Gotta also remember that some ashkin are said to be memories without souls or bodies. Bhoots, in particular, I think. In that way the Endless are very refined bhoots.