The fomor retreated to the deeps, where they had a profound advantage. Remember, they were a major power opposing the fae; and while Winter & Summer spent resources fighting one another, the fomor just built theirs up... for millennia.
When did Ethniu muscle in, take over? Dunno. Maybe they found her down there... maybe that was already her domain... maybe their retreat there was not just "to the deeps" but "to accept the protection of Ethniu."
When did the Outsiders begin collaborating? Was it only through Ethniu (or the Starborn Listen?) or was it more-generally with the fomor, perhaps Corb? Again: dunno.
But as best we know, the Fomor had millenia to build their intelligence-networks, to do their reconnaisance, to lay their plans.
And do not forget: the big apocalyptic Starborn event is about to arrive. The fomor know this as well as anyone. So they can be certain that there will be major disruptions happening, where they can take advantage, and advance their own agendas.
I think they likely had multiple plans & contingency-plans ready to roll, depending on where the cracks first appeared, which of the major player(s) weakened, or fell.