Did I skip something in the MSQ? This part doesn't make any sense to me

[CVXIV]

The way I view it is that once you become an Endless the memories of the departed are returned back to you, since at that point you would be part of the cloud with everyone else. This assumption is made because of our discussion with the man we returned his engagement ring to. It seems like the program attempts to create these 'chance' meetings so that the Endless can be happy and reconnect with those they lost. They don't know the pain of death because they end up seeing their friends and family again in this sort of afterlife.

Regarding why the warrior of light would be okay with shutting off the terminals has been a big debate, and people have argued whether or not the Endless are considered alive. In the end, we did not have enough time to reliably come up with another solution. Sphene refused to listen to reason no matter how many times we tried. We had to make a choice, and were presented one by Cauchia as a means to both free the Endless and to stop Sphene on her quest to perform interdimensional fusion which would have killed everyone. The system of Living Memory has always been presented as unsustainable, which is why Sphene worked with Zoraal Ja in the first place, for the souls promised to save her people.

To call back to Emet-selch's quote, "Remember us. Remember that we once lived." is something that still sticks with the warrior of light, and it is something we will do with the Endless. Of course we we never meant to see and interact with every Endless that exists, because that is unrealistic, but we will remember them for who they were just like we do with the ancients. People who in an act of desperation fueled by the countless death and suffering they faced turning to a way to preserve what they once had.

To bring up your question regarding Erenville, I think that the reason he didn't want to say anything is pretty simple, and it's that his role model, his mom, is dead, and he had been talking to a simulacrum made of her memories. I imagine most people would struggle to find the words to say, especially knowing that she wants the Endless to be freed, which would mean she would go away too. It would be a lot to take in at once, which would explain why he would go off by himself to think. It doesn't necessarily mean that the Endless have no value.

As for why the warrior of light goes along with this is because of what I mentioned earlier, the need to act. By the time we shut down the last terminal, Sphene is almost ready to begin interdimensional fusion which we know would result in the deaths of everything and everyone. I also don't think Wuk Lamat was ever given the role you've mentioned her having. Her homeland was attacked not just by Zoraal Ja's assault, but by the time bubble being erected on the outskirts of Shaaloani which essentially trapped her own people there which forced them to become Alexandrian citizens.

The reason Koana wasn't the one to come with us is because of what the two of them represent, Resolve and Reason. Koana stayed behind to better prepare Tural for invasion in the event our party failed. Had the roles been reversed, I don't see the sudden declaration by Zoraal Ja to continue the invasion going well for us.

Circling back to the Endless and the Alexandrian people, we do discover that they have records through the side quests. There is one where someone discovers a note and comes to the conclusion that the person who had written the note is their mother. However, they still do not have the memories of her or what she was like.

There is another side quest like this, that occurs after the attack on Solution 9.
I do not wish to share too much as to not spoil anything, but be sure to do the yellow quests if you want to learn more of the lore regarding the area.