I don't assume Eloise knows everything... in fact, a lot of what she knows, and what lies behind her comment that she is finally unsure of the future for the first time in a long time, is meeting her son (etc) in the past. That, plus the religion of the others, constitute her knowledge. Even if she truly believes "whatever happened, happened", she is not sure, and so she acts to make it happen as it had been told to her. I don't think she does this out of a desire to protect the integrity of history as such, but rather, a more particular reason. She, like Widmore and Ben, wants a particular side to win the war Widmore was talking about. I am thinking perhaps this war happens in 1977, or starts there more likely, and Eloise wants the side that won in her experience, to win again. That is... the others win that conflict, eventually by means of the purge. Eloise wants that to happen, she was (is) an other. Widmore might want that side to win (it gave him a lot of wealth), but also might want some other faction to win thinking there is even better opportunities for him.
In this case, Eloise sacraficed her son not nobly, as if to stop timespace from ripping apart, but merely to get the result she wanted. This is the "whatever happened, happened, or else god help us, where by us I mean those that won the first run through".
I consider this idea somewhat separate from the question of if history really can change or not, because it works in either actuality, since the characters cannot be fully sure history cannot be changed, since sometimes it hints at some change (seeming to put off someone's death for a day)... and this would be enough motivation to act even if someone was mostly sure it was not necessary. It's sort of a compatibalist view... she uses her free will to make the inevitable happen because, indeed, the universe makes it all fit together in the end, her lack of certainty about it give her room to be motivated, destiny happens because the illusion of will works towards completion of destiny. It is also of course compatible with the idea that stuff really can change, even if it takes special people and tends to self correct against attempts, requiring, for example, saving a man's life everyday until he finally dies of old age.