To get back to the OP, I think that while the Outsiders, Nemesis, or its agents might have long-term goals, like world domination, it's short-term strategies may change as events unfold.
For example, it appears Cowl made a deal with the Red Court to destroy the Senior Council for them. When that plan failed, it's possible the Outsiders or there allies decided it wouldn't be a bad thing for the Red Court to get hit by both Fairy Courts in order to wear the Red Court down, and make the Reds more dependent than ever on Outsider support.
Mab could have had more than one reason for holding almost all of her forces on the border with Summer. First, she would have wanted to keep the way to Arctis Tor mostly clear for the time when she would bring Harry Dresden to Arctis Tor to rescue Molly. Remember, there is evidence that the events at Splattercon!!! were planned about a year before the convention opened.
A second reason for Mab's behavior is more speculative. Mab may have not wanted the Red Court to get hammered by both the Summer and Winter Courts. She might have calculated that if both Summer and Winter attacked the Red Court, and the White Council later recovered enough strength to add its own weight to such attacks, the Reds might decide to sue for peace. I think that is something Mab would not have wanted to occur. I think Mab wanted revenge against the Red Court to come in a time and place of her own choosing way and she wanted her victory to be complete. Holding her forces at Summer's border ensured the Red Court would continue with its ruinous war.