Sion and Nihilus were her apprentices, so she was effectively leader of the triumvirate, although only to a certain extent. Nor did they really have individual powerbase, it was all one, they all used different parts of it and its almost impossible to divide it into clear chunks.
This is not really true. The group was called the Sith Triumvirate. A triumvirate is a government co-ruled by three heads of state. Each three of these had there own powerbases, agendas, and schemes. In fact, a Kaggath could be held between the members of this triumvirate. Traya was not "in direct control". The others cooperated with her and her plans, but as seen with their betrayal, they obviously were able to resist her command. Notice that its a betrayal, not a mutiny. Therefor Traya should only have her powerbase, or the forces of Malachor V.
Yes Traya was a co-ruler, but ultimately they were all 'Heads of State' and therefore the powerbase belonged to all of them equally, if not Traya that little bit more. Although ultimately it was Traya that told them what to do, else they wouldn't have usurped her to follow their own agenda. And where does it say that Sion and Nihilus 'betrayed' her? Not that that has much bearing, 'betrayal' was something of a buzz word concerning Traya.
But anyway, its impossible to have a fleet in orbit around Malachor V because of its sheer gravitational pull. Granted it is not strong as when the MSG was activated but nonetheless any large ship entering its orbit will be pulled in and crash. And that's before navigating web of debris surrounding it.
Granted a fleet could be placed at a distance from the planet, and deploy gravitation weapons, but that's not really going to have much effect. G0-T0 fleet is heading to the planet, so they can't really be pulled out of hyperspace as the drop out at that point anyway. And even if they are the ships won't detect them because they will be stealthed.