FATE tries to keep history from being drastically changed, not keeping it from being slightly changed. Hell, its mere presence in the past is a change in itself. Letting Porreans in and out isn't that big of a change.
From what I've read, Zeality believes that the fall of Guardia must have taken place in both Keystone-1 and Keystone-2 timelines. He backs it up with what I said above, FATE trying to keep history unchanged for the most part, to ensure its existance. Thinking Porre rose because of El Nideo, Elements, and Trade is a good theory, but I havn't seen anything to counter FATE not stepping in to prevent such a large change in time such as the most powerful country on the planet being overthrown. Well, maybe not most powerful, but you get the idea.