It seems Devereux only infects three people: Gregory Carlisle, the maid that’s part of her cult (Maria Sharma), and this woman, Charlotte Chapel:[1]
Gregory stays in his room, the maid stays in the small office, and no one else goes into either location. So if you take out Charlotte, no one else gets infected.
It turns out it’s very easy to stop Charlotte before she gets to the foyer: when you re-enter the library, go through the adjacent door to the foyer and up the stairs. You’ll be in plenty of time to shoot her or throw something at her head as she goes to the opposite stairs.
If you don’t stop her, it seems Charlotte can do one of two things: either she comes downstairs and stands watching the process in the foyer, only infecting a few people, or she comes downstairs, watches for a moment, then walks over to the other side of the foyer to infect most of the room. It feels like the difference is her AI getting stuck rather than intended randomness.
Even when she does get stuck, eventually one of the men she infects will join the crowd, and a few others will catch it from him, giving the Ether execution squad something to do.
Note there’s another woman who looks more or less identical to Charlotte. Charlotte begins in the foyer and doesn’t move. The other woman starts at the front gate and attends the meeting. ↩︎