I don't think the diversion argument really holds up, or at the very least, it falls completely flat the minute the Sath appears. The cruisers, corvette, and the Ravanna definately do ease the pressure on both the node defenses and the Bastion but, although never explicitly stated, I think it's safe to say that the Sath that attacks the Collie is the only one of the 80 plus fleet that takes a direct part in the Cappella engagement. All indications are that once the Sath fleet made it to Cappella they just went straight for the star; the fighting was left to the more conventional warships.
I mean even one Sath diverted to each node would end the battle in minutes. Revenge for the Collie's earlier Sath kill might really be acceptable as a reason for them bothering so much about it.