3) They ignored the rules. In this case the rules of behavior that guide supernatural interaction from breaking guest rights to showing off in front of humans to picking fights that should be impossible to win. It is often the case that individuals can achieve the impossible if they only refuse to believe it is impossible. This can lead to great goodness or horrible tragedy. The classic example is Hitler's Germany. Hitler's entire rise to power and much of their initial success had to do with Hitler's eagerness to make huge gambles by breaking the rules while using bluster, lies and legitimate grievances to stall or prevent the normal consequences. A house of cards of course - but if he have enough momentum it can be quite successful for a long time.
Don't ignore that Hitler came to power with the blessings of most of the people. They voted him into office, he managed to make economic and other improvements to their lives, then built upon that using their natural nationalistic tendencies, and last but not least deflected away from what he was really doing with the use of scapegoats. The Fomor built up over time, like you say, under the water, they kidnapped the vulnerable and little missed of the population that they altered over time to build up an army of powerful loyalists. Yes, they suffered a setback in the battle for Chicago, but
they are not finished by a long shot as long as there are those like Listen and his Turtlenecks who can blend into the population and cause havoc.. Even the White Council has bought into them being human enough that killing one violates the Law of Magic.