I do think the analysis looks sound, I do have other things to add that though. The Battanians are in a similar state where they can be attacked by many factions and are likely to be at war with more than one. Additionally, when fighting on two fronts losing two successive battles (and with that castles and/or towns) has a far larger effect, so these factions are indeed more easily steamrolled, simply via position.
As for the Khuzait is also very worth noting that here Geography plays a good role in defending, as the Makeb Chokepoint can easily be held against the Northern Empire, to the south Aserai would have to cross a day's worth of no man's land to get to the nearest town, while Danustica is indeed just around the corner.
Additionally Khuzait are cavalry centric and therefore faster on the map, meaning they can reach further, can retreat, chase and converge more effectively, especially since their home turf is also pretty dense, as all towns are within a day's or less reach. The Sturgians have more mountains and forests that will slow down the enemy, but more importantly themselves in a country that is basically one long stretch.
Also the Sturgians have the problem that their chokepoint against Battania and Vlandia is not secured by a town and that their town south of the mainland, which serves as a foothold, is hard to defend and therefore simply invites invasions.
I am also unsure, if the AI is always using geography to its advantage and if complicated geography actually hinders their movement more than the player. Generally AI scripts sometimes seem to complicated to be used correctly. For example Tyal was under Siege by the Khuzait and the main Sturgian force was too weak to defend or relief the town, so they decided to attack an underdefended castle in Khuzait mainland. Generally not a totally bad idea. However it took the Sturgians two days to reach it and another day to siege it. In that time Tyal siege was almost over (generally sieges seem way too fast) and while they managed to take the castle, they then spread out and got mopped up one by one by a way faster Khuzait army, leaving the Sturgians in a state worse than they were before or if they just tried to relief Tyal.
Something that should be considered is a 10% speed boost for Sturgians in the forests and mountains so they can coverge and retreat quicker as well as a general speed boost to all factions on their original home turf. Also the option for AI parties to make a makeshift defensive fort when being chases to level the odds a little. Like the wagon fort of F&S.