Well the Nemesis can take a Sathanas on its own as long as its not directly in front of it.So can the Hippocrates. Really, Sathanas is rather poor as a warship. It's only strength lies in it's ability to shock-jump enemies, anything not directly in front or directly behind it is relatively safe. At Sol, it'd have to approach a chokepoint that Sol Gate is, which would open it to attack from the sides. A couple of Lindos destroyers could probably keep up with them if they approached Sol gate like they arrived from the 3rd Knossos portal. Sol Gate, having only one arrival vector, would make a neat ambush point.
The only reason the Sathanas is poor as a warship is because it has noticeable side armaments whatsoever.
On other subjects, I rather like how the EA's motivations can be interpreted. I've always imagined from the transmissions the GTVA would've sent after Capella, Earth would be viewing the GTVA as either too unstable to defend the Human and Vasudan species effectively, and would view their capability in this capacity as suspect. Not just due to Shivans, but because of the NTF Rebellion and ****.
It gives them something better than "lol we're evil", and adds a story element that can be built upon. Rather than just "hey hey we're going back to earth, man!"