The Carnor Jax era was always one that will be known but still the collapse was inevitable.
The thing is, if R2D2 and Lando hadn't of hacked the Eclipse II's overly automated navigation systems-sending it on it's fateful collision course with the Galaxy gun destroying both and Byss in the process, it would have probably orbitally bombarded Onderon after the Emperor's final death-likely killing Luke, Han, Leia and the few Jedi in the galaxy as well as the Physician and Royal Guard there.
This would have left Byss, the Eclipse II, the Shadow Hand Fleet and the Galaxy Gun all in the Imperial Ruling Council's hands by default-I.E. Jax's hands. With the power to back up his claim to the Throne the Warlords likely remain at least nominally loyal, the New Republic at this point is on the verge of collapse having lost nearly all it's territory and most of it's military to Thrawn and then palpatine's offensives. In this scenario there is no witness to jax's treachery, so Kanos and the remaining Royal Guard have no reason to declare vendettas against Jax, some might even have SERVED him and with access to Palpatine's Dark Side Compendium Jax could well have become a mini Vader(since he died fairly early in his Force training and Lumiya was not fully trained herself we'll never know just how strong he could have become in the Force.)
In this scenario the Galaxy likely does go back to Imperial Rule under Jax.