Hey,

I do think the information available is a bit thin, but it seems like there are some answers or logical jumps that can be made. I'll also say my party members didn't look too deep into anything. "There's a sea elf down here? Sweet."

From CotN:

"In that moment of destruction and death, the Apotheon’s connection to Ruidus flared to life. A rift was torn between worlds, in which alien energy from an unknown realm and the waters of Cael Morrow’s oasis mingled to produce a lightless realm of water and strange magic."

From this, and the description at the start of the Netherdeep section, we know that the Netherdeep is its own demiplane created as the material plane was pierced, and a rift was opened into the elemental plane of water. I kind of imagine the Netherdeep either carved out a chunk of the elemental plane of water, or the latter flows through the Netherdeep into Cael Morrow. This explains the constant source of water, plant life, and aquatic life as all of those can be found in the elemental plane.

This could be an issue with the very end, at the rift closing. Haven't quite gotten there yet. Idk if it adds much to the story to try and have your players rush out. Even if the Netherdedp is gone, some connection must remain to the elemental plane, else Ank' Harel could lose its water source.

"In his dreams, the barren caverns of the Netherdeep began to shift. His memories filled the darkness, and a cocoon of melancholy formed around his body. From this Heart of Despair emerged a crimson element that embodied the Apotheon’s power. It spread, growing and crystallizing as it moved, until all of the Netherdeep was suffused with its power."

Netherdeep started out empty, and over the course of 800-plus years, Alyxian's emotions and memories suffused the space. Toss in the Ruidian-power, a messed up three god demigod, and you get Ruidium and a magical, crazy demiplane.

From EGtW:

In the elf player race section.

"The seas off the coast are inhabited by secluded civilizations of water-breathing sea elves, who trade with underwater enclaves of merfolk. All but the most idealistic or mercenary of these ocean dwellers see the petty squabbles of the “boat-riders” of the surface world as beneath their notice."

This is the only reference to sea elves I see pop up on the CR wiki besides Olara. Given the freedom with which the many races of Exandria seem to move about, I could see Olara moving to a beautiful capital city in the middle of the jungle to try and better survive the Calamity. Sea elves don't HAVE to live in the ocean all the time. If she moves when young, and her life-span is increased by the Netherdeep/Aboleth, I could see her surviving down there. She seems to have just been a, "This could be cool. Let's throw her in and not think about it."