1) Again, they don't really care about their losses. If losing a Moloch means taking down an Argo, the Shivans are fine;
2) But the Shivans realized that there was no opposition when they entered the system. They chose their assault force at least a few days before. And they couldn't monitor Terran or Vasudan movements in other places than the nebula;
3) Because the attacking force, though small for Shivan standards, was fearsome for the GTVA;
4) We can't be sure of the fact that there were 80+ destroyers. The immense number of spacecraft at the disposal of the Sathanas fleet was enough to irritate the GTVA. If there were so many destroyers, why do we see only two of them in the last missions? A Demon or a Ravana could have attacked the Bastion, for example. And we know that the Bastion's group was decimated by fighter and bomber wings, possibly launched from the Juggernauts.
Command would have hardly accepted the risk of sending in the Bastions with 80+ Shivan destroyers lurking around. The Shivan fleet had two purposes: to destroy civilians and to engage as many GTVA forces as possible, so that the Supernova would have done the rest.