Having given the matter some thought, I'd be interested to see what people think of the following structure:
In the Provisional Contract, ducal authority is vested in a number of named individuals who collectively embody the office of duke; here, on the other hand, the dukedom is vested in two special fiefs- I have called them 'limb baronies'- which enjoy special prerogatives.1. The Dukedom of Brookshire is vested jointly in two ‘Limb Baronies’: Greater Lakhesis and Overdolor.
2. The territorial integrity and autonomy of the Limb Baronies is guaranteed under the Contract of Governance.
a. Each Limb Barony is free to establish its own system of governance in accordance with local tradition.
b. The lord or administrator of a Limb Barony possesses ducal authority and prerogatives within zir territory. The Ducal Government is not permitted to interfere in the internal affairs of a Limb Barony.
3. Each Limb Barony nominates one individual to serve as a Ducal Limb.
a. The senior of the two Ducal Limbs is the Emissary of the Duke and is styled, ex officio, Marquess or Marchioness of Modan-Lach. The Emissary represents the Duke of Brookshire in the Adelsraad and on any other matter requiring the Duke’s presence.
b. The junior of the two Ducal Limbs is the Marshal of Brookshire and is styled, ex officio, Baron or Baroness of Hamlet. The Marshal is the deputy of the Emissary and exercises zir functions should the latter be unable to do so.
c. The seniority of the two Ducal Limbs alternates on a weekly basis.
4. The two Ducal Limbs collectively embody the will of the Duke of Brookshire.
a. Decisions or edicts issued in the name of the Duke must be endorsed by both Limbs in order to take effect.
b. Should the two Limbs be in disagreement, then the matter shall be decided by a vote of all resident nobles of Brookshire, with the majority opinion prevailing.
5. The Contract of Governance may be replaced, altered or otherwise amended only with the agreement of both Limb Baronies.
Why do I think this is a good idea?
Firstly, it is more flexible that the present structure. Consider Greater Lakhesis...under this system, Malliki could be the baron and Spraki the limb, but with scope for one to stand in for the other should that prove necessary. The actual Contract need not worry about contingencies for every circumstance precisely because many of the details are decided by the individual limb baronies.
Secondly, this structure gives the limb baronies a large measure of protection and autonomy. Their territory is guaranteed by the Contract, they are able to choose their own lord or system of governance in accordance with local custom and most importantly of all, within their borders, their lord or administrator is effectively the duke. In other words, no-one save the Kaiser (and presumably the Adelsraad) can interfere with their internal affairs....which should help to reassure the inhabitants that some idiot isn't going to come along and ruin everything just because they can! In turn, I would argue that this might actually help to maintain the integrity of Brookshire: people will be less likely to rebel or secede if they known that their autonomy is fundamental to the functioning of the duchy.
Anyway...I could ramble on and on, but it makes more sense for interested parties to read my points and then present their opinions and/or concerns.