Hypothetical Surrender Scenarios.

[KaelanFrey]

Maybe overthrow leader and put a new one that's cooperative in whatever Bretonia may demand in the future.

This sounds like the most likely hypothesis.

The Coalition is an immense threat to the Houses, but to space-nuke them or get them to cooperate on their own* would be counter-productive. One would waste all the precious space vodka and the other would result in the Coalition buttcloaking the Alliance again.

Either annexing the Coalition worlds or turning them into puppets with a new government & military under constant surveillance would work.