Book spoilers: The biologist's husband is dead before the biologist joins her expedition. However, she doesn't believe the husband that comes back is anything but a copy. She joins her expedition with some intent to find her real husband, but also because she is fascinated with nature, and is really quite a loner. The biologist that comes back is a copy. The real biologist stays in Area X and retraces her husband's footsteps. She doesn't find him, but there is a hint that he may have been transformed into a dolphin she sees in the first book, or maybe an owl that she lives with in the third book. The biologist resists being transformed for 30 years by hurting herself (the Area X infection heals her body instead of transforming her). Eventually she gets exhausted by her life in Area X, and allows herself to be transformed into some kind of giant sea/land going beast. Time flows faster in Area X, but later characters find the biologist's diary/last statement.
There is no real need to bring Area X back to the real world, as at the end of the second book, Area X expands catastrophically, and opens other areas on the Earth, one where the copy of the biologist is hiding. There is an implication that this happens because of the presence of the copy, and that the current expansion of Area X is not quite civilization ending, but could well be in the future. Area X is some kind of terraforming machinery or disease from a long dead race, but travelling through the doorway takes you to what may be a different planet (different stars). No one knows what happens to those that pass through the border rather than go though the doorway. They may get transformed, the may just disappear. The disease was released from the old lighthouse lens and infects the lighthouse keeper. It was done deliberately by a black-ops in plain sight project who don't realise what it will cause.
The only time the words annihilation are used is because that is the hypnotic keyword that allows the psychologist (who is also the Director of Southern Reach) to compel the expedition members to kill themselves. The psychologist tries to use it on the biologist after the psychologist is mortally wounded by throwing herself off the lighthouse, but by then the biologist is immune to hypnotic suggestion from breathing in the spores in the Crawler's writing in the tunnel. The crawler used to be the lighthouse keeper before he was infected and subsumed into the alien terraforming disease.
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