Thread quickly became discussion about ancient empire, so i will write about something different to bring some fresh air.
GTA was close to actually win conflict by military measures - or at least strike Zods hard enough, to cripple PVN ability to continue war. Vasudan military-industrial complex capabilities were much lower than terran, and they were forces to use Anubis, despite having better fighters simply because they could not afford to produce more comples fighters in large quantities. Also, Zods overextended in prievious phase of the war, launching several deep strike operations (remnants of these are Vasudans in Ross 128 or Betelguse), and many of their battlegroups were either destroyed, left without enough supplies or badly damaged. Terrans were in the meantime preparing to launch decisive offensive, using GTD Hades to directly strike one of Vasudan core worlds and military shipyards. Fighters and weapons we see during FS1 were actually created to be used during that offensive - new heavy fighter Hercules to replace Apollo bomber type, and Medusa bomber with tsunami warheads were in my opinion created to combat PVD Typhons. However, the GTA was rotting and internally collapsing due to failing war-time economy and civilian unrest caused by massive losses and suspension of some liberties. After eventual victory over Vasudans, these problems would probably cause collapse of GTA - also, GTI would probably try to seize power and create dictatorial state.
Bosch rebelled because... GTVI was close to find out what he is planning. Before NTF rebellion Bosch was probably one of most influential military officers (in not the most one), also having strong political and business connection. It would alloud him to redirect large amount of resources to his projects (like construction of Iceni), and gather more data he needed. However he was simply too big fish, and Intelligence started to check his activities. He could not finigh ETAK in secret in time he had left, so the only choice was to start an uprising.
Why te rebellion lasted 18 months, and then collapsed so quickly (even without Colossus it was quite obvious, that it is only matter of time, before NTF would be crushed. In first few weeks/months terran component of GTVA fleet was in a state of constant mutiny - entire squadrons and warships simply defected to the NTF instead of fighting them, further swelling their ranks. Also, Bosch had large number of agents or followers of NTF ideology in GTVA military, conducting acts of sabotage or gathering intel. GTVA had to conduct wide "purges" to eliminate remnants of Bosch clique in its own ranks. Later, GTVA simply did not tried to solve war quickly, fearing of inevetable losses - they simply decided to wait for deployment of Colossus - Bosch also knew about Colossus, but being not restricted be neccesity to keep low-profile he had enough time to finish ETAk.