The Fomor were definitely more active after Changes as a direct result of the Red Court's extinction. But whether they had actually been asleep before or just kept in check by the Red Court is probably just a technicality at this point.
Why not both. There are a lot of agendas out there that are both aligned and in conflict. The reds and the formor are not some minions of the black council and/or outsiders. They are allies - or dupes - or manipulated - or victims - or ... well you get the picture. But the key point is that each of these supernatural races are acting against the White Council because they see advantage in doing so. Advantage specific to their personal position (of the leaders), and their nation/species.
It is perfectly possible that the Formor are both helping the Reds and coveting the Red's territory at the same time. And it is perfectly possible that the Formor have "jumped the gun" from the outsider's perspective by trying to claim so much territory right now and stirring up the supernatural world before the outsiders are ready to support their efforts. Or maybe they are very closely aligned with outsider objectives. I am not sure myself.
One of the real nice elements of the Dresden series is that this is not a conflict of monoliths. The Red Court fell in part because of internal feuding. The White Court refused to support the war against the White Council due to their internal feuding. The White Council's response to the war was inconsistent/hesitant because of their internal feuding. The Summer and Winter Courts are not working closely together despite both playing a vital role in protecting the outer gates. This disunity at times helps and at times hurts the cause of humanity. But it always makes for more interesting stories.
The whole "game" of the outsiders is focused on manipulating the various supernatural nations into fighting - and distracting - each other, helped by a dose of nemfection to manipulate a few key players. None of these groups would believe they are doing the bidding of outsiders. I doubt even the black council believes that.
