I think you should add a bit more for realms staying under the 1/3 total noble limit.
You can probably combine this with the hinterland idea. Giving those staying under the limit ability to travel faster within the allied territory.
Giving incentives to stay under the limit can probably be called 'highly cohesive' state of alliance.
Alliances can specify a variety of levels of support: for instance, whether members will aid each other in combat only in defense within member realms, defending in neutral territory, defending in enemy territory, or even joining in to attack together.
We can probably apply the same concept here as well.
defending in member realms - low cohesion (Realms with >30% of total noble count)
defending in neutral territory + defending in enemy territory - high cohesion (Realms with <30%)
If people even under the new system choose to form a mega alliance, it might be a good idea to add some extreme stuff like allowing realms staying in the highly cohesive state to cash bonds in any city within the alliance.
