What if Kerensky wasn't the savior, but instead, knew about it.
The units that were left in the Hegemony were those that were against Amaris and/or Kerensky.
Kerensky took the rest of the troops out of the Periphery, to take out Amaris' competition.
Kerensky sends troops back not in attack patterns, but back to the hegemony to help with defense.
The rest of the SLDF completely conquerors the periphery nations (of course, the RWR was quite easy).
Then the SLDF picks a house, and goes after it since it was mostly intact.
200 years later, the SLDF is reborn, owning everything, with an extremely high level of technology, far less loss of life.
It isn't like the inner sphere thought the young Cameron was a great leader. They had already started distancing themselves from the hegemony and all feared Kerensky and the SLDF would take them out anyway.
Isn't too much of a stretch for Kerensky/Amaris to have gotten together and planned the whole thing. Execute 1 person, and his lineage, destroy a few loyal military units, all to save the inner sphere and the star league.
In history, bad guys are only bad guys because they lost and good guys are only good guys because they won. Had Amaris won, he wouldn't be the Hitler of the 28th century, but the Churchill instead.