Humanity will Take a Third OptionThe human race would be most useful to the Umiak as they apparently are resistant to Loroi telepathy, not to mention that two of our worlds are of the type preferred by them. As far as we can tell the Loroi treat their allies and subjects much better than the Umiak do but are not afraid to commit xenocide. The emperor might decide to destroy humanity before the Umiak can enslave them. Then the Barsam and some other Loroi allies will come to humanity's aid, the newly formed alliance will also liberate some of the races subjugated by the Umiak.
Humanity will side with the Umiak at some point.The Umiak are not a nice people, enslaving their "allies" in all but name, committing what would be war crimes on Loroi civilians (if both races weren't both about to exterminate each other), and even treating their own people as literal cogs in the war machine. But the Umiak are pragmatic in their own alien way. They have yet to waste precious resources xenociding third parties when they can conquer and add them to the war effort. They're willing to call for a ceasefire even when they outnumber the Loroi. Life under the Umiak would be miserable at best, but Humanity could survive it. The Loroi have already wiped out two species, but the Umiak may be convinced to let Humanity live in servitude rather than spend time and warships killing us all.