Finally finished the game.

Ending spoilers:

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The aliens do care about the volunteer - whenever one of your squad goes down you get a comment expressing concern that if one of the chosen one's pack went down so easily then maybe they were wrong, and if the volunteer goes down you get that nonstandard game-over and the aliens are dismayed that their candidate for ascension has failed after all.

Which makes the last moments even more baffling; clearly they expect your team to survive until the final room, since they're using the gauntlet as exposition about the races they tried to uplift. And they have more to tell you in that room. So okay, they attacked as one final test of your ability or something, confident that their new candidate could handle it.

But then what is going on with the last room? They swing with lethal force, and if they succeed, are unhappy because their candidate died (and you even get a special message about how upset they are that you were so close when you went down). But if they fail, they freak out and tell you that you're doomed without them to guide you? What outcome were they expecting, that you would counter the assault and force the enemy to obey you without killing them?

Actually... I was wondering if there might have been an alternate ending if you kill the other enemies and not the super-Ethereal. It doesn't sound like there is, but maybe you're supposed to mind-control it to prove yourself, or something? I doubt there's actually a secret ending, but it would make marginally more sense... pop psi inspiration right before triggering the fight, trigger and mindfray as much as you can, and then try to mind-control it? But you'd have to do that in a single turn, since once it acts it'll mind-control someone and become immune...