We have a new head of state line, and they've been doing a pretty bang-up job about it.
From the shattered and disparate remains of a once-proud successor state to launching wars of conquest and rebuking multiple hostile invasion attempts simultaneously in the space of a decade. When did the pre-Jihad FWL manage to do even one of those things at a time? I'll give you a hint: literally the Age of War. Six hundred years ago.
Well in answer to your question take a look at mid to end of the Second Succession War after the Comstar war concluded "La Reconquista".
Look here is my problem with the fig leaf of Marikhood of Nikol and sibs. This isn't the case of a Greco-Roman adoption where they were trained for the position by the adopting parent. This isn't an adoption where Thaddeus Marik was a parent to Jessica's kids and raised them as his own. It lasted like 5 minutes/months they had no real interaction and then he was killed off. Them being Marik's is a polite fiction in universe, those who are deeply allied to them pretend it has legitimacy but know it won't hold up to any kind of scrutiny.
Throw in the fact that Thaddeus himself was from the Republic and Nikol has done nothing to secure the Covenant World's loyalty to herself directly, essentially treating them as just another member state it essentially limits her claims to being the heir to Thaddeus Marik.
House Halas has a long history within the FWL Nikol and there is nothing wrong with being a Halas they should just be themselves and be proud of Oriente.
I mean I have some thoughts on some of the steps in Shattered Sphere but Im going to leave that out of here because some people might not have picked it up yet.